Sailor Earth



"If an NPC is known as the "One" I cannot volunteer to be the "Two"."

- Things Mr. Welch Can No Longer Do In An RPG

In stories where the characters are organized with a kind of official order, hierarchy, or theme it becomes easy to simply expand that group to add new characters of the same type. This means the writer doesn't have to Retcon half the Backstory in order to add additional characters to the cast. Likewise, well-done fanfics can use the same principle to insert original characters, bending Canon instead of breaking it. If this is done to simply add a new character out of the blue, it can give the illusion of added legitimacy to the new character's place within the hierarchy/theme of the cast. If the main characters are the last of their kind, then these characters will be There Is Another.

Unfortunately, Mary Sues appearing as Original Characters in Self-Insert Fic using this trope are common enough that some fandoms have a knee-jerk hatred of them. Especially when the trope is used to rehash ideas for a role that has already been filled in canon or previous fanfics. However, Tropes Are Not Bad and it can still be used legitimately.

Named for a particularly prevalent phenomenon in Sailor Moon fiction.

See also: Send in the Clones, Elsewhere Fic. If a character of this type becomes Mary Sueified, and then overhauled with more power than the canon should give them and then some, it becomes a Copy Cat Sue. See also The Unchosen One. Not to be confused with the webcomic Sailor Sun.

Please do not add personal examples of this to the page. This is discussing general trends of the trope or particularly notable examples.

General

 * Pretty much any series with a school/classroom setting will have hundreds of fics introducing a New Transfer Student, and a smaller but still significant number introducing a new teacher, often an aged-up original cast member or someone from a different series.
 * Space Operas make it quite easy to introduce another spaceship with a captain and, perhaps, a full crew, though unlike most examples, this is, by definition, very reasonable.
 * Any series set in a military environment can easily accommodate team changes that ensure a steady influx of new fan characters.
 * Similarly, hospital dramas can always add new nurses and interns.

Anime & Manga

 * Any hypothetical Sailor Soldier in Sailor Moon, but especially the obviously missing "Sailor Earth" (whose role is technically filled by Usagi's boyfriend). The manga's creator later admits that none of the later senshi were actually planned out before they appeared, since the series was initially intended to only feature the few girls to follow the Five-Man Band rule; the "every planet could have a senshi" Shrug of God eventually became canonical and used frequently in the anime and especially musicals.
 * An early appearance by a Sailor Pluto rather than a Saturn just fueled the early speculation there should be senshi for three leftover planets; Fanon versions of them existed before canon versions.
 * After the Sailor Moon anime, whose Sentai ascendants were likely to have literal Sixth Rangers, magical girl shows needing filler usually have an easy mnemonic for adding new characters when needed.
 * Note that pretty much any astronomical object tends to get Senshi, and this was going on well before Pluto was demoted from planetary status. The series itself was mum on where the Sailor Starlights actually got their Sailor Crystals from (though the anime implied through a flashback they may have been representing moons of the planet Kinmoku), and Sailor Cosmos isn't helping any. Stars are extremely common targets for fan Senshi, the most prominent continuing to be creating a Sailor Sun (aka Sailor Solaris if you're being technical). Also, there's precedent for galaxies to get them if you really want a Mary Sue.
 * What's annoying is that many of the fan senshi do not stray too far outside the box. Although the Senshi's fuku, or uniforms, are shown to be vastly different around the galaxy with only the Sailor collar being the always present component (compare the Starlights and the Solar System Senshi), many OC creators tend to stick with the Sol Senshi uniform formula despite their being little to no evidence of Senshi elsewhere have similar uniforms.
 * Dragon Ball fanfiction loves to introduce previously unknown members of Goku or Vegeta's family, like a long-lost sister or a half-whatever-hybrid left behind during the Saiyajin wars of conquest; forgotten childhood friends are another popular one. Ditto Namekians. Additional Cyborgs aren't as popular, since the Word of God has listed all of Dr. Gero's known creations, but this doesn't stop some from appending more onto the list.
 * Incidentally, Vegeta having a long lost sibling was Sure Why Notted -- see the 2008 special.
 * Lunch has a sheer amount of daughters that always go along really well with Goten and/or Trunks.
 * Speaking of Trunks, another popular theme is to have Goku and Chi Chi have a daughter and have her hook up with him.
 * Sometimes it's Gohan and Videl's daughter, Pan or a self-inserted Mary Sue.
 * Given that the Saiyans are established to have regularly been sent all over the universe for conquest (and Goku being sent as an infant was not an unusual case), the idea that one more could've slipped through the cracks of Freeza's massacre is quite plausible. That it would happen to be a member of the royal family? A bit less so.
 * Any number of new Nth Children in Neon Genesis Evangelion. Nth Angels where N is greater than 17 also crop up in Continuations whose authors ignore both endings of the series.
 * Oh, and let us not forget Eva Unit N whose major difference from the others will be production country, color scheme and placement of (any) horns. Fans are divided on whether Evangelion Rebuild is canonizing or deconstructing this trend with Mari.
 * In the Rozen Maiden anime, there is another dollmaker named Enju, who only made one doll. He is often seen as a plot device with which to bring in more dolls without affecting the Rozen Maiden lineup.
 * Tokyo Mew Mew makes this easy; there's an almost infinite amount of sweet foods and a lot of endangered animals.
 * Since only a small fraction of the seated shinigami officers of Bleach have been revealed in the Canon, it's easy to introduce new characters, especially in the less-known divisions like the first, the seventh, and the tenth.
 * This also applies to the numbered Arrancar. 110 of them exist, but only about 20 of them have been shown, much less named.
 * There are also the rare Visored Sailor Earth, Quincies who thought they were the last of their kind before meeting Uryuu, humans like Chad, or other types such as variations of Quincy Powers (Since there's an entire world to explore and Bounts could possibly not be the only spiritually-aware humans), or other types of people with Kido-shinigami powers like Orihime and Hachi. Of course; these people are often very rare - possibly because these types are often rather harder to do than simply naming one-self as a seated officer or arrancar who wasn't introduced.
 * Rarely, one may actually have a Bount who wasn't rounded up by Kariya.
 * May also apply to Uryuu -- when he was introduced, it turns out he had been in Ichigo's class the entire time. Ichigo just didn't recognize the name.
 * The extreme ease (in both canon and filler) at which it seems anybody (regardless of whether they appear to be obviously the wrong age) can transfer into Ichigo & company's high school makes it that much simpler to introduce such an OC into the main characters' circle of friends.
 * Bleach fanfic writers also tend to enjoy adding in a "14th Division", which allows them to make their OC a captain without killing off any of the existing ones. Meaning that three already vacant places are not enough for some.
 * And now there are Fullbrings, humans who can have pretty much any power because their mothers were attacked by hollows while they were pregnant with them. This is pretty much a blank check for any fanfic writers.
 * In Fruits Basket fic, there is the 'wolf of the zodiac' story. Despite the fact that the Eastern Zodiac only has 12 slots, with the cat added from tradition, many ficcers will add in another animal, usually a wolf or some other charismatic animal. This character will almost always be a girl who falls in love with Yuki and/or Kyo.
 * Much of Yu-Gi-Oh!! Fanfic consists of inventing a new Millennium Item and a bearer to go with it. This rarely ends well.
 * Parodies of this fandom cliché tend to have the Millenium Spork or something like that.
 * This happens in the card game as well. Example: Millennium Shield.
 * Not all Sues in Yu-Gi-Oh are Millenium holders; sometimes they're lovers/sisters/daughters of canon characters. (I'm looking at you, Pharoh; you must really get around.) A lot of the time someone's daughter/sister, someone else's lover, and have an item!
 * If Seto is the main character, expect the new character to be his new secretary. Alternatively, she's his new maid or Mokuba's baby sitter, but that doesn't happen quite as often.
 * Joey has no special qualities yet often fights in important duels. Any duelist can join the fight pretty easily, with the only special thing about them is their actual skill.
 * Pokémon fanfics involving aspiring trainers just starting out on their journeys... and written in a way that makes the canon human characters look like incompetent chumps. On the other hand, that isn't difficult to achieve, as writing characters with sense puts them miles above the canon cast, or at least Ash.
 * There's also an infinite number of newly introduced Team Rocket members. Those with a little more creativity just invent their own "Team Whatever" instead. Given that a new "Team Whatever" (or more than one, in some cases) is created in canon for every new region, this isn't surprising.
 * This may also apply to the huge number of invented Pokémon people create. A few of them are creative enough, while most of them are not. A fan favourite would be made-up Eeveelutions. In one notable case, fans came up with Leafeon, right down to the name, far before it was introduced in canon.
 * Addressing both of the above, one site somehow managed to come up with not only them name Team Plasma, but nearly the same GOAL they had... in 2008. Site here.
 * Suzumiya Haruhi: Either have a new member of the SOS Brigade be the ever-elusive Slider who hasn't shown up, or have Haruhi watch some movie and get interested in... vampires, wizards, robots, pirates, whatever.
 * Given the Claymore system of ranks and the fact that we haven't seen all of them, it's easy to imagine a Claymore who hasn't been seen yet filling one of the vacant numbers. However, despite what some fanfics claim, there is no Claymore #48, since it's explained in-story that the Organization that makes Claymores specifically only assigns one to each of 47 regions of the setting making a #48 a clerical error of epic proportions. As one snarky reviewer of a fanfic of this sort said "And what region is she, then? The sky?"
 * Jo Jos Bizarre Adventure makes creating new Stand users simple: pick two musicians or bands, name the user after one and name the Stand after the other. You don't even have to make the Stand's power correspond to the name in any way. For safety's sake, however, stick to music from over ten years ago; we don't want you to be stoned for a Stand named "My Chemical Romance", do we?
 * The third series, however, is slightly more difficult, as the protagonist's Stands follow the pattern of "TarotCard Color". Finding legitimate-sounding tarot cards can be quite a challenge, as "Puce Devil" can't possibly be anything other than pure evil. On the other hand, you can name your character Freebird and his stand Master of Puppets, and you're good to go.
 * Now would be a good time to note that only the Major Arcana of the Tarot deck and the Egyptian pantheon were canonically used in Stardust Crusaders. This opens up the potential for the Minor Arcana (assuming you're using a Tarot that gives them individual names), Court Cards, and other pantheons. It also adds serious potential for entire societies of stand-users based on the suits of the Minor Arcana or said pantheons. At the very furthest stretch, using the alternate names of Major Arcana cards could also work.
 * Believe it or not, even the colors of the protagonist Stands are allegedly music references -- Hierophant Green, Magician's Red, Hermit Purple, etc. Star Platinum is supposedly a reference to a record going platinum.
 * After the third arc, even the heroes have purely-musical Stand names -- Josuke has Crazy Diamond, Giorno has Gold Experience, and so on. Anything goes.
 * Though randomly adding a fifth member ruins the numerical symbolism present in Weiss Kreuz, Weiss have gone through so many new teammates they're practically the Rolling Stones of assassin teams -- and Schwarz aren't doing much better. The one canon team which has proved largely immune to the sudden influx of new female teammates is, ironically enough, the one team which was all-female to begin with.
 * Notably, when Weiss does get two canonical new members in sequel series Weiss Kreuz: Gluhen, they both, making the scads of random new members who show up in fanfic that much more nonsensical.
 * Any given series of Digimon has a group of kids with Mons and Digivices, so generally it's a simple matter of picking a partner mon and a Digivice colour. Bonus points if the Digimon partner is entirely made up.
 * For Digimon Adventure, which already had an arc searching for a Sixth Ranger dubbed the Eighth Child, Sailor Earths often pop up under the title of the Ninth Child or higher.
 * This is easiest in Digimon Adventure 02, which had a whole "World Tour" arc introducing kids with Digimon partners from outside Japan, and had the last episode state that, by the time of the Distant Finale, every single person on Earth has a Digimon partner.
 * Once upon a time there was at least one multi-author Fan Verse solely dedicated to 02 and the exploration of this trope in conjunction with Elsewhere Fic.
 * Also rather easy in Digimon Tamers, in which they could have been another tamer whose wishes were granted, or their partner Digimon actively sought them out.
 * Digimon Frontier also has four semi-canon (potential) Sailor Earth positions to be filled: the four remaining Legendary Warriors. You know, the Psycho Rangers from the first half who never made a Heel Face Turn? At least one fanfic has their spirits given to humans (in this case, all four of whom are somehow related to the heroes from Adventure/02).
 * That's very rare. Most Sailor Earth fics add another element, despite that breaking a lot of ideas a reader can have about elemental balance...
 * The fact that Advanced Hybrids do exist for Izumi, Tomoki, Junpei and Kouichi (JetSilphymon, Daipenmon, RhinoKabuterimon, and Reichmon respectively) but never got to be used, opens the door for Day in The Limelight Interquel chapters to showcase them.
 * Digimon Savers fics tends toward new members of DATS; it helps that it's mentioned in passing that other DATS units operate around the world, including the Austrian one Touma came from early in the series, so filling the gaps in in those is pretty easy.
 * Digimon Xros Wars fics generally involve the creation of a human general, a lead partner Digimon, and an accompanying Digimon army, though there's no indication that other human-led armies operated at any time during the war against the Bagra Empire. Digimon Xros Wars the Young Hunters Leaping Through Time offers a more solid foundation - it's clearly mentioned that Digimon Hunting is something of a sport with many participants, so having another Hunter is pretty believable.
 * Zoids, in the New Century and Fuzors series at least, involves an entire league of teams battling for fun, profit and prestige. How many fanfics have started with the main characters battling a new team? The same could be said of nearly any other competitive sports show out there; Beyblade, Crush Gear Turbo, Battle B-Daman...
 * New Card Captors with either their own Clow Cards or another set of Cards in Cardcaptor Sakura. They tend to ignore the canon facts that 1. the Cards were a new form of magic invented by Clow; and 2. the Final Judgment couldn't take place until all the Cards were sealed.
 * In Ojamajo Doremi, Sailor Earths often pop up as the green Ojamajo. Similar to the Organization XIII case, a green Ojamajo, Nozomi, actually showed up in the OVAs. The clue? Ever since then, a good chunk of Doremi fanfiction has revolved around
 * Which pretty much screws Canon, since it was stated that . But most people tend to ignore it.
 * Ojamajo fancharacters almost invariably show up as green, and named "Fasola," in order to establish some sort of Theme Naming after Doremi. If not green, expect rainbow, or a mix of two or more colors, which will form an alternating pattern on their dress. Bonus points for black and/or red (even though, like, there IS a red Ojamajo - ) or if they are somehow related to Onpu.
 * Chobits fandom is rife with extra Chobit-like persocoms, either as actual extra Chobits.
 * The hosts of tailed beasts 3 and 5-7 in Naruto. That a cover celebrating the manga's 9th anniversary give designs for all of them despite just encourages this.
 * Don't forget all the extra tailed beasts people come up with later, e.g. the ten-tailed wolf. Or the people who seal the yin half of the Kyuubi into another host.
 * The fandom also seems to be in the habit of inventing other members of the Seven Swordsmen of the Mist, of which only Kisame, Zabuza, and Chojuro have been mentioned. The more intelligent ones preserve the Theme Naming and the odd swords. The less detail-oriented, and the filler writing staff, don't. Too bad all of them have showed up in the canon.
 * For that matter, not only are the 5 major villages relatively little-known, but the canon map displays a dozens of tiny countries that could house new minor ninja villages (the filler writers certainly made good use of this).
 * Not that there isn't plenty of Sailor Earth material in Konoha itself. Just have a character be from some second-tier clan with some "(Japanese word)-gan" Kekkei Genkai. Or even make them a long-lost Uchiha or Uzumaki clan member.
 * Until her identity was actually revealed, there were a reasonable number of fan-created red Cures for Fresh Pretty Cure based off of the fourth petal of the clover logo. Since said clover also had a green center, the idea of a green Cure or Cure-like ally was also popular.
 * Heartcatch Pretty Cure saw a few yellow and purple Cures (possibly due to the logo featuring yellow and purple in addition to pink and blue)...but only briefly, as Cure Sunshine and Cure Moonlight's appearances were leaked very early on. There is still Sailor Earth potential -- in this continuity, there were numerous Cures who had their own adventures long before the main heroines did, though they only appear in-series as anonymous statues.
 * This was even done in-universe! When Potpourri was searching for the third Cure, he encounters Ban, who is trying to come up with a Sailor Earth of his own, Cure Fire, for his manga. When Potpourri suggests he be the third Cure, he starts imagining himself as such. Potpourri runs for the hills and Ban uses Cure Sunshine when she appears.
 * One Piece features such cases as the third cousin once removed from Garp's side of the family, or brand new Straw Hats that have the "Whatever Whatever fruit".
 * For years, the fan-made crewmember was almost always given the job of ship's musician. This was thanks to a Running Gag in the series where Luffy really wanted a musician for his crew, but could never find one. Fan-made musicians were typically very beautiful women (often mermaids) who played the harp or the violin with otherworldly skill (similar to Conis, who did not join the Straw Hats). The series put a stop to this when the canonical Strawhats finally got a musician, who is not a beautiful woman but a jovial dead guy. He does play the violin, though.
 * These characters still pop up in One Piece fics all the time, though; they're simply given a different job, often one that is made up for the fic (such as "helmsman", even though the crew has never expressed the need for one and usually just take turns at the helm). This likely stems from an early (and possibly outdated) statement by Luffy that he wanted ten crewmembers, and Jesus Burgess' position in the Blackbeard Pirates.
 * Back before all the Warlords of the Sea were revealed, it was common for some OC villains to be this. This was back when Mihawk and Crocodile were the only ones revealed to be one and actually be made as characters, where Doflamingo and Kuma, while revealed, had little screen time. Other popular positions were Admirals and other high-ranking Marines.
 * Particularly Egregious when a Supernova OC will be introduced to "The Eleven Supernovas." Some authors will change the name to "The Twelve Supernovas," kill off an existing Supernova, or ignore the name of the group completely.
 * Ranma One Half has no fewer than four very distinct plot elements to do this, all of which see regular use in fanfiction. The list is presented in no particular order and often they can combine into one mega plot device:
 * A martial artist with a random martial art. Given the way the anime devolved into an Opponent of the Week format, a new character with an appropriately thematic martial art style can literally wander into the Tendo Dojo to challenge Ranma. Even the manga allows for this, though not quite so easily.
 * Ranma has another fiancée. Yet another spurned fiancée wanders into the Tendo Dojo to enforce her claim on Ranma's hide, with a stolen dowry/contract backing up her claims. Thanks to the Noodle Race episode, which features a new fiancée showing up at the beginning of the episode and implying that Genma has done it more than once, there's plenty of ammo for a new character to be introduced. Again, even the manga provides for this with Ukyo (on Genma's side of things) and Picolett Chardin (for the extremely rare cases when a new fiancé for one of the Tendo girls shows up in fanfiction).
 * Ryoga has a long-lost brother/sister/aunt/uncle/cousin. Ryoga explicitly states he rarely, if ever sees members of his family... and genuinely believed he had a sister near his age when Ranma tricked him with a disguise. It's not much of a stretch to assume he does, in fact, have family readers have never before seen.
 * The Amazons/The Musk/The Phoenix Tribe. While the Amazons are the ones most often abused for the creation of original characters, each of the three provide for the character having some sort of mysterious background with the strength and martial prowess to be an appropriate match for Ranma.
 * A fifth such plot element is the cursed training grounds of Jusenkyo. Pantyhose Taro's curse of Drowned Yeti-Riding-A-Bull-While-Carrying-An-Eel-And-A-Crane and Rouge's curse of Drowned Asura give them incredible fighting power, so an inventive author could use this all by itself.
 * Fullmetal Alchemist's Sailor Earths usually come in the form of homunculi with a randomly gothic-sounding name with the author trying to pretend it actually fits into the Seven Deadly Sins Theme Naming, or State Alchemists in their teens proficient in a ridiculous and completely useless field of thematic alchemy, completely disregarding the notion that State Alchemists are supposed to be people of mass destruction. God only knows what use the Amestrian army has for alchemists with CodeNames like the Starlight Alchemist or Angel Wings Alchemist. They also tend to forget that Ed is, canonically, by far the youngest State Alchemist in history, so bringing in one around his age is highly implausible. The distant third contingent is the Xingian royalty. Since it's stated that Ling has at least twenty siblings, his father has fifty wives, and we only know one of his half-sisters, Mei Chan, it's pretty easy for anyone to whip up their own prince or princess without much effort. This has never been as popular as the two first options, mainly because it doesn't apply to the better known first anime.
 * Since the characters of Axis Powers Hetalia are all Moe Anthropomorphisms of countries, it's very easy to create an OC by selecting any nation that hasn't shown up in canon; you can even opt to make a province like Canada's Quebec or The United States' states into a character. You're expected to do some research to make sure that your OC reasonably fits that nation/province's stereotypes and history, though, and inventing a country out of whole cloth just reeks of lazy writing and/or Self-Insert Fic.
 * Many people liked to put someone in for the missing North Korea, until a blurb on Himaruya's site was finally translated, stating many important details about him.
 * Shugo Chara fandom is rife with young girls who have over 3 charas, even if the series explicitly states that usually people have 1 or 2 at the most. Their response: 'Well, Amu can have three, why can't Sue?'
 * Soul Eater takes place in an Extranormal Institute and only focuses on three students at that school, leaving the rest of the students open to any fanfic writer's desires.
 * Seven if you count the weapons (who do indeed have to attend classes and take exams.)
 * Katekyo Hitman Reborn is a story with only the most prominent mafia families in that universe. Also, the Vongola might have quite a lot of enemy families. One can easily whip up their own family with some Italian-like name, and then fill it with many original characters (could be all girls). Then the story could be about Tsuna and co. meeting this family or getting attacked by this mystery family, resulting in allying with another original family. Also, many tend to disregard the fact that the point of having seven colors, seven flames, and seven core family/team members were to be a representation of a rainbow and its seven colors, so many authors write that Tsuna's family ends up expanding his family with more rings, and more colors (usually black or white, but sometimes an obscure color). Usually with more rings comes more characters to take on the role in the family. While some authors are a little thoughtful about the meaning of the positions they create for their characters, others only do it so the character is present and hooks up with one of the guys.
 * Actually having more than seven rings and seven colors was sorta canon, as said in one of the games, but there was a specific reason why the topic of an eighth ring isn't touched upon.
 * The Vindice seem to be the only known characters in canon that have access to the eight element, and it seems to be so powerful that
 * Konjiki no Gash Bell is a favorite with this trope because it allows for double Sailor Earth. Since most of the demon children on the run for Demon King were never introduced, it's no wonder fans write in their own Demons and Book Masters in periods were it was possible. Others goes as far as writing about another competition Twenty Minutes Into the Future despite the main character's motivation being ending said competition.
 * Though, at the end of the series, the main character Thus, a subsequent competition isn't completely out of the question. In fact, who is to say that participants in past competitions had not come to the same realization?
 * Though rare, Yu Yu Hakusho fanfics may involve a future spirit detectives (since it is not stated whether or not Yusuke was succeeded by another teenager), another participant in the Dark Tournament or king's tournament, another student who wasn't taken by Dr. Ichigaki, a past student of Genkai, or someone from Mushyori city who became spiritually aware when the demon gate was opened (since Genkai stated that there were a lot more people who came to her after developing powers than just Kaito, Yana, and Kido).
 * Inuyasha Sailor Earths may be another human, demon, or hanyou who was cursed by Naraku or has another bone to pick with Naraku for some reason. Naraku does have a Zero-Percent Approval Rating, so it is only natural everyone has a bone to pick with him.
 * Or you can make a Sailor Earth as one of Naraku's new incarnations or spawn, since he seems to pop a new one out every fifty chapters or so. He's had a dozen or so with various Elemental Powers, so creating a new incarnation doesn't take much imagination.
 * The real Sailor Earth in this series is when fanfic authors give Inuyasha another sibling (oftentimes a sister). Sometimes with their own super sword from their father (that is often even more powerful than the swords he left his canonical sons). The problem with this is there WAS A THIRD SWORD (movie three featured Sou'unga, the Sword of Calamity). And it was indeed more powerful than either of the other two (if they tried to take Sou'unga alone).
 * Inuyasha, the hanyou whose father had many daughters when he was already dead. Then again, some people in this anime seems to have a problem with staying dead.
 * Bandai Entertainment did this to their Inuyasha RPG adaptions. The Secret of the Cursed Mask for Play Station 2 has either a boy or a girl and Secret of the Divine Jewel for NDS has an american transferred student Janis, all that are pretty much Kagome's Expy upon their introductions. While they have their own plots, the whole point is only to have their original characters go on a quest along with the Five-Man Band.
 * Ouran High School Host Club fanfics seem fond of introducing a new member of the Host Club out of nowhere.
 * Another popular tactic is to make a Hostess club and create five or six new characters at once.
 * Mermaid Melody Pichi Pichi Pitch...oh, good luck finding a fic that doesn't introduce a hitherto unknown "red" mermaid (despite the fact there are no oceans left for a mermaid to represent... but then writers can get creative with things like "the Bering Sea" mermaid). If it's not red, colors can start from white and can get extremely ridiculous, to the point of having "metallic colored" mermaids and mermaids with the color of "tangerine" and "mint"...
 * Other fics choose to take the fact that
 * Justified because as the second season demonstrated, an ocean can't survive without a princess for long.
 * Still doesn't stop fanfic writers from making the new princess Super Special Awesome.
 * Death Note has Wammy's House, with 26 letters (or the combined total of all alphabet letters) of Sailor Earth goodness, for your crazy detective needs, usually a crossover of Copy Cat Sue of L or other canon Wammy attendants.
 * The number of unused Three Kingdoms or even Dynasty Warriors characters in BB Senshi Sangokuden, coupled with the immense amount of leftover Gundam mecha provide prime fodder. Xiao Qiao Methuss, Cao Zhi Airmaster, Cao Ren Leopard...
 * For Kamichama Karin, all you need is another Greek god. Some people don't even stop there and start recruiting gods from other mythologies as well. Oddly, there seems to be an even split between making new characters and just giving the existing ones new powers.
 * Tenchi Muyo! fics tend to fall into this trope. Either a new girl joins Tenchi's "Harem" who naturally has superpowers of her own (to varying degrees of awesomeness) and usually develops feelings for Tenchi too, coming into the usual conflicts with Ryoko or Ayeka. OR add a male character, with super powers of some sort (natch), usually to be a boyfriend to one of the various girls at the Masaki household, since Tenchi has so many of them lying around.
 * Magical Girl Lyrical Nanoha, from StrikerS on with its Mildly Military setting, allows for the existence of other TSAB teams and units that can act alongside the canon characters.
 * It's very easy to make a magical girl for Puella Magi Madoka Magica, what with the billions of girls on earth to be contracted in past, present and future, and the revelation that Kyubey's kind visit other planets as well only made it easier. The only real requirements are  Hell, one could even make another  if they wished.
 * And now that Puella Magi Kazumi Magica has revealed Juubey, people are running with the idea of them being a numbered series.
 * it is also fairly easy to create your own Witch. There is pretty much an infinity of them all across the ages, with an infinity of gimmicks, new ones appearing every day. There's even a template all laid out for you by the official descriptions: a name, a mindscrewy design, a nature and a short description for the witch herself. Another name, design, duty and short description for her minion. Given that "birdcage witch" and "class representative witch" are valid Witch gimmicks (both appear in the anime), then you can see Magical Girls can never run out of enemies to fight.
 * Thanks to Mobile Suit Gundam's rich history, especially in the Universal Century, it's not impossible for some hotshot rookie who is a Newtype/Cyber Newtype/Gundam Fighter/Coordinator/Innovator/what-have-you pilot some super awesome Gundam to fight the evil forces of Zeon/The Titans/Neo-Zeon/Crossbone Vanguard/Zanscare Empire/OZ/ZAFT/A-Laws/what-have-you. It doesn't help that, in the One Year War-era, we've seen tons of fronts of the Federation/Zeon war and that, in the Mobile Suit Gundam SEED universe, Gundam isn't actually the name of the awesome Super Prototype - it's the name of the OS. Everything there is a Gundam!
 * And now that Puella Magi Kazumi Magica has revealed Juubey, people are running with the idea of them being a numbered series.
 * it is also fairly easy to create your own Witch. There is pretty much an infinity of them all across the ages, with an infinity of gimmicks, new ones appearing every day. There's even a template all laid out for you by the official descriptions: a name, a mindscrewy design, a nature and a short description for the witch herself. Another name, design, duty and short description for her minion. Given that "birdcage witch" and "class representative witch" are valid Witch gimmicks (both appear in the anime), then you can see Magical Girls can never run out of enemies to fight.
 * Thanks to Mobile Suit Gundam's rich history, especially in the Universal Century, it's not impossible for some hotshot rookie who is a Newtype/Cyber Newtype/Gundam Fighter/Coordinator/Innovator/what-have-you pilot some super awesome Gundam to fight the evil forces of Zeon/The Titans/Neo-Zeon/Crossbone Vanguard/Zanscare Empire/OZ/ZAFT/A-Laws/what-have-you. It doesn't help that, in the One Year War-era, we've seen tons of fronts of the Federation/Zeon war and that, in the Mobile Suit Gundam SEED universe, Gundam isn't actually the name of the awesome Super Prototype - it's the name of the OS. Everything there is a Gundam!

Comics

 * This happened to an explosive degree in Green Lantern's Sinestro Corps War, with legions of ring wielders for every color either coming into existence or being hinted at.
 * Sodam Yat. The longstanding fanboy desire to see what Superman would be like with a Green Lantern ring was satisfied after a fashion by giving a ring to a Daxamite. At first he was simply an exceptional ring wielder, then the Oans revealed to him that yellow sunlight would give him Superman-like powers and as his body was charging with solar energy the first time, they gave him the Ion entity which took his Green Lantern powers to the next level.
 * There was actually a Vulcan Green Lantern once, who appeared in a page of an issue of the series in the 1970s. While unnamed, the character even makes the Vulcan Salute! He was just sight gag/ShoutOut at first, until the current Continuity Porn-filled run brought him back, named him Saarek, and gave him a gimmick: he's apparently a medium/necromancer.
 * And let's not forget the Pierson's Puppeteer GL seen in the Larry Niven-authored graphic novel Ganthet's Tale; though most likely simply a Shout-Out to the author on the part of artist John Byrne, the thought of a fearless Puppeteer (because this was before the recent retcon to the GLC's primary qualification) makes the mind boggle. And Byrne drew it with a neat mane, too, so either it was so insane that it came out the other side into sanity, or a Power Ring can be used as a hair care tool.
 * To say nothing of the fanart of just about anybody and their brother (anime/manga/game/TV/movie/book characters included, plus non-DC comics!) with the color of ring most suited to their personalities.
 * Also, in spite of the fact that this theory has been allegedly Jossed by Geoff Johns, the idea of the colors uniting to form the white light of creation to combat the Black Lanterns has led to the creation White Lanterns in many fandoms. The lead up to Brightest Day only fueled it further.
 * Common fandom jokes: Brown Lanterns, aka the Apathy Corps. Also Infared and Ultraviolet Lanterns.
 * This was extremely easy in X-Men fan fics. Everything from new members, new students at the school, even entire teams could be dreamed up easily, since the only criteria is that one was born with his powers (and even that isn't set in stone). This is harder to pull off in the current continuity, however, after the whole M-Day fiasco. Unless you just pretend it didn't happen, which many fans would like to do.
 * Grant Morrison revealed in New X-Men that, in the Weapon X program Wolverine belongs to, X is actually the roman numeral 10. He then introduced Weapons I through XV.
 * The title character of 10th Muse is, you guessed it, the tenth daughter of Zeus and Mnemosyne - the muse of justice (because, you know, comic book). Her archvillain? Mnemosyne's only son.

Film

 * The setting of The Matrix lends itself extremely well to original characters: just have someone new get unplugged from the Matrix. In fact, you can make a whole ship full of OCs if you want (preferably with a classical name, in the tradition of the Nebuchadnezzar and the Osiris). In most cases, the fact that the vast majority of redpills are unplugged well before adulthood will be addressed in one of two ways: if the fanfic writer is an adult him/herself it'll be overlooked, and if they're a teenager or younger it'll be used to justify making the character their own age.
 * The League of Extraordinary Gentlemen fandom is rife with fics picking up after the end of the movie (less so the comic) which introduce a new member to the League. Said member is 99% likely to be young, female, beautiful, and possessing a power not already present in the cadre. Lycanthropy is popular, but so are various Psychic Powers. Often this character will have some connection to one of the canon characters, like a previously-unknown daughter or sister; one example that comes to mind is a fic which gave Mina Harker a sister who was a werewolf; not only did the sister also have the last name of Harker (Mina's married name), but the novel Dracula explicitly identified Mina as an orphan with no living relatives.
 * Real Steel introduces a world where boxing is now done between robots. Each robot has a different theme, and many of them are implied to be homemade. All a fanfic writer would need to do is create a robot, and give it a Backstory. The film and additional material related to it give many facts for World Building as well; for example, robot boxing originated in Japan.

Literature

 * New Harry Potter classmates, especially ones that they would never let be enrolled at Hogwarts in the books or movies. This could include such canonical characters as Blaise Zabini, whose existences were known but who had never appeared "on screen".
 * Students (or former students/graduates) of Beauxbatons or Durmstrang may also show up.
 * And what about exchange students from The Salem Witch Institute? Obviously common whenever an American fan wants to do a self-insert fic, with a variant being to use some of the canon characters as exchange students to an American wizarding school.
 * There is also the common variant of Japanese exchange/transfer students, thanks to the frequent crossover between the anime and Harry Potter fandoms. (Especially common on Gaia roleplaying guilds.) Some RP groups address both problems by explicitly banning Hogwarts student characters who were not born in the British Isles.
 * Percy Jackson and The Olympians is constantly prone to a new demigod, which is made extremely easy by the ending of the fifth book
 * And the sequel revealed . And then the sister series gave us not only characters   and an Ancient Conspiracy, but pretty much confirmed they all exist in the same 'verse.
 * And even further, Word of God has stated that the next series will feature the Norse Gods
 * Tenth Members of the Fellowship are surprisingly common, even though this overrides the nine Ringwraiths = nine Fellowship Members motif put out in the book. Really Sueriffic writers will quickly note that for opposite numbers there were nine Ring Wraiths... and Sauron!
 * Then when people give you that answer just smugly say "And Elrond!" It Makes Sense in Context.
 * In James Bond fanfics, there will often be a character whose codename is either 00x or a letter besides M and Q. This is not helped since the movies have, at various times, either shown or mentioned 3 different 006s, 3 different 009s, 2 different 002s, a 003, a 008, and even a 0011. Most of the other 00 agents that have more than a passing mention in the movies tend to wind up dead shortly thereafter. It's very clear that not only do numbers get recycled when their former owners die, but that any given number works as long as it's preceded by 00 -- making 0013, 0069, 0099, or even 008675309 all valid code-names.
 * Makes sense; 00 is simply an agent status, showing they have license to kill.
 * Combined with the Epileptic Tree that James Bond is merely a code name, there exist plenty of fanfics starring a brand new Bond after the previous one was killed/retired. At least it's better than ones about Bond being a Time Lord.
 * Mildly lampshaded in the film version of Casino Royale - James to M: "Well, I understand double 0s have a very short life expectancy..."
 * The Warriors fanbase is notorious for this. Most Warriors fanfics have no involvement with the canon characters (either the writer will create new Clans or write about the book Clans in a different time period than canon). In fact, it's so incredibly common that it's often seen as Mary Sue-ish for a fancharacter to exist amongst the canon characters.
 * Quite a few Animorphs fanfics have somebody new join the team. This plays out interestingly, cause the Animorphs already had a Sixth Ranger, but he sold them out to Visser Three to get his parents back. So, naturally, there is some skepticism in the group if they want to take in another new member, Touched by Vorlons or otherwise. But it gets resolved within a chapter, after the Original Character promises not to kill them in their sleep, and Marco mentions he needs a pairing buddy.
 * Another obvious mechanism: Vissers. Vissers Three and One are recurring villains, and with 47 Vissers, only a few of whom we've met, it's easy to bring in a new one.
 * Alternamorphs, the Choose Your Own Adventure spinoff media, actually embrace this trope. In the second one, the reader even plays as the aforementioned Sixth Ranger.
 * There were also the Auxiliary Animorphs who were mostly people who were not suitable controllers (due to being disabled in some way), and a fanfic can easily put the new character in the shoes of one of them, sometimes with varying disabilities. (A good example would be somebody who is Autistic and wouldn't be suitable because diferent behaviours would alert non-controllers that something is up.)
 * There can also be another faction of Auxiliary Animorphs who were deemed unsuitable controllers for other reasons - such as they are children and are too young to be of any use as of yet, they're too old, or they do not have the proper social status to become a good controller.
 * The Clique has a common theme in their fanfiction: A new clique that resembles the original one comes in and competes with the Pretty Committee. The new clique is either lead by a Massie-esque bitch that happens to beat Massie when it comes to hurling insults, or a total sweetheart.
 * 99% of fanfiction for Mary Hoffman's Stravaganza series involves the introduction of a new Stravagante. Which, in fairness, is how every book in the series starts.
 * The Star Wars Expanded Universe uses this quite often, usually in a good way.
 * Obviously, at the end of Return of the Jedi, Han/Leia is standard, but what about Luke? Enter Mara Jade.
 * Who helped Leia get the Death Star plans? A former stormtrooper turned mercenary turned Jedi Knight named Kyle Katarn.
 * Luke needs Jedi students, so... Kevin J. Anderson gave him a dozen.
 * In the Fix Fic I, Jedi, in addition to that dozen, Mara, and Kyle, we have Corran Horn.
 * The surviving Y-wing pilot from A New Hope is canonically you in the X-wing games.
 * So we have Jacen, Jaina, and Anakin. They need friends to join in on their adventures, plus show romantic subtext. Hence Tenel Ka, Zekk, and Tahiri, respectively. There's also Lowbacca.
 * The Railway Series has The Island of Sodor and Sodor: Reading Between The Lines which suggest that there are eighty locomotives on the Fat Controller's railway alone, as well as a number of engines formerly working on Sodor and several branch lines, minor railways and industries that could be rail-served. Fanfic characters are therefore usually just a case of finding a gap in the Sodor stockbooks and dropping an engine (often the author's favourite class of locomotive ) in. In many cases, as real locomotives have been turned into characters in the series proper, creating a character is often as simple as putting a face on a real engine and inventing a reason for them to come to Sodor.
 * Most Holes fanfics have someone new coming to Camp Green Lake - and it's almost always a girl, despite the fact that it's an all-boys camp. This is rarely even handwaved.
 * Lost Targaryens in A Song of Ice and Fire fanfic who just happen to stumble on dragon eggs and start conquering stuff.
 * There are a lot of noble houses in Westeros, but only a few of them have names: the rest are only mentioned by their heraldry. This allows for would-be Sailor Earth makers to create their own house by making up a coat of arms or taking one of the many heraldic designs described in the book. The Song of Ice and Fire tabletop RPG includes this as an option in its character-building process.
 * In The Hunger Games, there are 73 past Games, meaning 73 opportunities for fans to invent their own victors (who are inevitably Mary Sues).
 * The Dragonlance series has an interesting inversion in the official Tales trilogy; a historian claims to have discovered evidence that there was a gnome member of the original 'Heroes Of The Lance', who was written out of the Chronicles and his existence completely hushed up by the rest of the heroes.
 * With Dragonriders of Pern fics and RP fora, this tends to manifest itself mainly in four ways - all of which have some basis in canon:
 * A dragon outside the five standard colors (female Greens and Gold/Queens, and male Blues, Browns and Bronzes); one that isn't a sport or a runt like Ruth (canonical White dragon). Often written to have special abilities (the ability to talk physically as well as mentally being a popular one).
 * Female Bronze/Brown riders and male Queen riders.
 * Riders with Journeyman Craft ranks (Usually Harpers)
 * Children of Lord Holders (Pern equivalents to princes and princesses) becoming riders.
 * The majority of Maximum Ride fanfics and R Ps features a new mutant or group of mutants that has broken out of a lab. The former always joins up with Max's flock and the later usually becomes thier own

Live Action TV

 * Any series with a Monster of the Week formula, like Smallville, makes it easy to introduce a new superpowered character.
 * This is easier in Power Rangers than you might think. Considering nearly every team leaves at least one common ranger colour out and the canon Sixth Rangers are allowed to join up late too, all it takes is to pick a season, pick a colour they didn't use, throw a little Deus Ex Machina in here and there and voila! The White Ninja Storm Ranger! The Green Dino Thunder Ranger! The Black Mystic Force Ranger! And that's without even getting into non-canon ranger colours...
 * Parodied in S.P.D in which one of the minor characters lied to his parents, claiming he was not just a ranger, not just the leader of the Power Rangers, but the first ever Orange Ranger. Ironically, Super Sentai did once have an Orange Ranger: Battle Cossack from ''Battle Fever J."
 * With the government/military-based teams, it becomes a very simple thing to have a random joe in the organization get promoted and saddled with a blank power template. This is especially the case for S.P.D., which canonically had several squads of Rangers already.
 * And let's not forget entire teams created to fill a Power-Rangers-Equivalent of Sentai teams preceding Kyoryu Sentai Zyuranger.
 * Or the five normal Dairangers who never got their suits used.
 * Some even go ahead and make future Ranger teams based on future Sentai teams; currently, Gokaiger is one popular choice.
 * Thanks to the number of additional mecha that get piloted remotely, it's easy enough to design new Rangers after them. Canon already has the Titanium Ranger (whose colors don't even match his Max Solarzord, but still counts) and the Jungle Fury Spirit Rangers. In toy form, we have the hideous repaints of the Operation Overdrive Rangers that come with the bikes, and three original Rangers for the Paleomax Megazord of Power Rangers RPM. Those are just the tip of the iceberg when you consider that Power Rangers Wild Force by itself has up to 94 unassigned Wild Zords...
 * In fact, they sorta do this with the second set of weapons, based on each Ranger's secondary Zord.
 * The possibility of a seventh or eighth ranger has been part of MMPR since season 2, but especially season 3; many fanfics make Kat the Green Ranger so Kimberly can stay on.
 * Many fans seem to enjoy purple as the eighth ranger color. This is actually Ellen Brand's doing to some extent; she created a "meaning" to the colors. This was all parodied in Joe Rovang's fanfic, "The Neon Gray Ranger: Damn, What A Man", which focuses on the silliness of Mary Sue.
 * This later became Hilarious in Hindsight when Gekiranger was adapted into Jungle Fury, only the Violet Ranger was the fourth one to appear.
 * There's also the Rangers or Ranger-like heroes that were in the Great Offscreen Wars and Cryptic Background References - previous Wild Force, Mystic Force, and Samurai teams existed and so little is known of them. Lotsa fanfic fodder there.
 * There are several teams in Stargate SG-1 that have been mentioned but never seen, leaving a lot of wiggle-room for fans. This one especially is really easy, as not only is Stargate Command just a US Government agency instead of some weird group of Destined Ones, "SG-1" itself is just a serial designation. Want a full roster of OCs? No problem, welcome to SG-7. Making it even easier is the fact that there are at least 9 SG teams in canon from the very first episode, up to at least 25 by the 10th season, with the audience getting to see what SG-1 is doing 99% of the time, with the occasional appearance to SG-2 or SG-3 members. That's at least 100 potential characters. Add to that the fact that the teams are at least somewhat specialized, and you've got even more to play with. And that's not even counting the people who are almost certainly going to add SG-0 as a secret black ops unit that's better than SG-1 but who nobody knows about.
 * With the upcoming Stargate MMO, it seems likely that players will get to take their SG-Whatevers into the virtual plane. Watch as clans of avatars take on even the toughest of Goa'uld! Of course, video games are generally all about that.
 * Let's not forget the Goa'uld themselves. Even with the necessity of excluding the Norse pantheon, there's a vast number of mythological deities who were never used in canon. That's a lot to pick from if a fanfic writer wants an OC villain for their OC characters to confront.
 * The same thing happens with Torchwood, for which the main characters for series 1-3 worked for "Torchwood 3" -- though canon gives explicit designations for 1, and 2, 4 has vanished and is easily brought back for fanfic purposes, and it's not entirely ridiculous to imagine there's Torchwood 5, 6, e, -90423, or what have you..
 * It used to be that you could at least insist Torchwood was solely based in Britain. Following the introduction of Torchwood India in the radio drama "Golden Age", it's quite reasonable to assume there are branches all over the former British Empire, as Torchwood was founded by Queen Victoria.
 * As noted above, Space Opera shows can get this. Especially common is fanfic about Starfleet, since we don't even know how many ships there are. At an absolute minimum there must be hundreds. There are many ships that the main heroes cross paths with for one episode, and it's easy to crew them with O Cs.
 * It's also possible to do it with ships, as they have themes. Need a new DS 9 runabout? Think of a river! Need a new ship the same class as the local version of the Enterprise? Think of a modern battleship!
 * Kamen Rider has the potential in spades, especially with some series that leave the possibility open or even pointedly declare that it could happen.
 * Agito explicitly says that all humans have the potential to become Agito.
 * Ryuki had this in spades, the existence of Abyss (see below) adding even more to it, as it was possible that any of the massive number of Mirror Monster could become an Advent Beast for a new Rider.
 * Faiz had six riders taking six Greek Letters, leaving eighteen more. There was also a stageshow which featured Kamen Rider Alpha, Beta, and Gamma.
 * Hibiki gained also has opportunity, giving existing character Akira the ability to transform into Kamen Rider Amaki, and other side characters becoming Oni. Not to mention that there is more than one Takeshi branch.
 * ZECT was constantly producing new Zecters. All one needs to do is think of an insect.
 * Den-O's Imagin are based off Fairy Tales, so it's possible for one to make one based off an unused fairy tale. Kamen Riders in that setting are mostly made by bonding an Imagin to the suit, so you can also plop a pre-existing Imagin to make a new Kamen Rider.
 * The Imagin Sieg was supposedly based on the story of Swan Lake, which actually had a black swan Evil Twin named Oderjean to the protagonist, white swan Odette - hence, this pic of Sieg's Evil Twin.
 * Kiva has tons of lore waiting to be fleshed out, like the First Kivat.
 * Interestingly, Kamen Rider Ryuki gained a canon Sailor Earth in the form of Kamata/Kamen Rider Abyss, an original character made for Kamen Rider Decade who uses Mirror Monsters that were seen but not contracted in Ryuki.
 * Also, Decade has Kamen Rider Kivala, empowered by a small, female Kivat-like bat-critter from the World of Kiva. She didn't operate in the World of Kiva,, but she's what a Sailor Earth looks like in canon.
 * Double is super easy, as nearly any rider can be made out of any noun.
 * OOO had three Black Core medals which were not used by Eiji. Several fan-art created what would happen if Eiji used them for a combo. There were also made up Core Medals and rarely Greeed to go along with them.
 * Monsters in the Kamen Rider franchise are easier to make. Especially with the Heisei era Kamen Riders, in which one can just select an animal motif that wasn't used before in that show. There are a few exceptions (like the Worms having only insect motifs, or the Imajins also need to be based off a fable), but the largest exception is Blade, in which all 52 monsters are accounted for. However, it's easy in Double, where one just needs to think of a noun, and bam, instant Dopant.
 * Some Rider series also make it easy for new forms to be created.
 * Kuuga? His forms all came to be because Yuusuke was thinking "I need to be able to do X!" and when it got really desperate, his suit turns a new color, gains a power... and loses one. To come up with new forms for him, you only need to think of a color that hasn't been used. (Red, white, blue, green, purple, black, and now gold - Decade does it again! - are taken. Anything else goes. You can also give "Rising" upgrades to your new forms, and give "Amazing" upgrades to new and old forms - if there's Amazing Mighty, why not Amazing Titan? Amazing Ultimate?)
 * Den-O? Just add another Imagin, original or existing.
 * Double? You don't even have to invent new memories from whole cloth. What if Philip and Shotaro used another Rider's Gaia Memories? Or even Dopant Memories?
 * Also, Fourze introduces new Astro Switches via the "Hey, look what I whipped up today!" method, so new attachments for him can be easily introduced.
 * Used to be common to insert a new survivor into Lost. In fact, the show itself inserted two entirely new crash survivors into the plot during season 3, linking them to several significant events in their sole flashback. Not so common after seasons 5, which
 * Cutey Honey the Live has three girls based on three suits of cards. You can easily come up with a new one to represent clubs. In fact, the two other protagonists are a canon example of this: Honey's always had heart designs, but interpreting it as the suit hearts and coming up with diamond and spade girls with the same power is new.
 * While this didn't really work during the show's run, the Buffy the Vampire Slayer finale created hundreds of Slayers around the world. As a bonus, Slayers are always teenage girls (except for those who manage to make it to their twenties), and as Most Fanfic Writers Are Girls...
 * Angel firmly established that the Slayer being the sole warrior for humanity was a load of poppycock, with numerous individuals and organisations working against (and for) "vampires, demons and the forces of darkness", creating massive fanfiction potential.
 * Happens four ways in Doctor Who fics:
 * In classic series fics the Doctor would run into any amount of former classmates and Time Lord renegades who nicked their own TARDIS and legged it, semi-justified as it often happened in canon with the Rani, the Master, Drax, the Monk, etc.
 * In new series fics the introduction of the Chameleon Arch allows for any amount of Time Lord survivors hiding in the local populace.
 * In fics for both eras the Doctor would land the TARDIS and pick up a totally-not-a-self-insert-at-all new companion.
 * For the vast majority of episodes, the only regulars are the Doctor and his one-to-three companions, featuring otherwise entirely new casts. Fanfiction can and has easily followed this formula.
 * Many Glee writers have yet to realize that adding a thirteenth member to New Directions throws off number balances, as there is usually an even split between boys and girls (Jesse didn't help that lesson sink in much either, even taking into account that his stay was temporary).
 * Do what they did in Season two: get rid of Matt Rutherford.
 * Season 2 ended with them having 13 members, but then again Blaine can act as a counterbalance despite being in a different school.
 * At the end of season 3, they have 16 members, though 8 of them graduate.
 * There's also an indeterminate number of nameless, faceless Troubletones girls.
 * Supernatural has had a specific brand of this since pretty much the beginning of the show: the sister-fic. Just make up a name and give her a reason why she wasn't seen in canon up until that point (hidden by John for safety, taken by demons because she's psychic like Sam, etc.), and boom! You have a female Winchester. Never mind the logical quagmire she often creates--how can she be younger than Sam, but still be Mary's daughter? Why would John let one child live a normal life, and not all of them? Some writers try to avert this by just making the fic an AU where she's always been there, and is very frequently Sam's twin. Usually, the new sister ends up being a Mary Sue, and the stories are just rehashes of episodes where her presence makes everything better.
 * Another common occurrence in the Supernatural fandom is to create one or more new hunters, often a set of sisters. In the Supernatural universe, this isn't impossible, since hunters exist across the world. While some people create interesting additions to the universe, most are blatant self-inserts, especially because if these characters are female, the boys seem to fall for them by the end.
 * Every now and then, Criminal Minds will mention other, unseen BAU teams. One of them actually was showcased in the Season Five episode The Fight.

Music

 * Neither Earth nor Pluto appear in Gustav Holst's Planets Suite; Pluto hadn't been discovered yet, and Earth was, from Holst's astrological perspective, insignificant. Some amateur composers have created music to fill in these two gaps.

Tabletop Games

 * This is outright encouraged by Warhammer 40000. Want to make up your own Space Marine chapter? There's canonically a thousand of them, and only a hundred or so have been so much as mentioned in canon, so go on ahead-- it's even perfectly valid to imitate an existing army due to the way successor chapters are created. There are also two Primarchs whose identities and abilities are unknown, seemingly for this trope. Similarly, there are a million worlds in the Imperium that your homebrew Imperial Guard regiment could originate from, the Eldar are so secretive that a previously unheard-of Craftworld suddenly appearing wouldn't be out of place, and the Tau are always settling new Septs. And so forth for Dark Eldar Kabals, Tyranid Hive Fleets, Ork tribes, Necron Tomb Worlds, Chaos Space Marine warbands... there's always far more of them than are ever actually detailed, and there's no shortage of players willing to make a neglected bit of background their own.
 * The Chaos Codex takes this concept Up to Eleven. Not only can you invent your own renegade chapter, you can also mix and match various legions and chapters into one army, with the justification that they are all united under one banner for some cause or another. This was originally the concept for the Red Corsairs, before they had their combat doctrine retconned.
 * Exalted recently made room for not one, but two separate Sailor Earth Exalt types in their recent Alchemicals splatbook. Not only did they open the possibility of Exalted with a natural affinity for Adamant, they also said that the Abyssal Exalted are a mistake and not intended, leaving it open for a new Exalt type who has a "true" affinity to Soulsteel. These two types are only mentioned as vague possibilities--nothing to suggest what they should be, if they exist. It was suggested in Dreams of the First Age that slain Incarnae existed once, and their Exalted went mad and were wiped from existence; this is one potential explanation of the above. However, given the source and given that it is generally considered to be quite feasible--if far from easy--to take on one's almighty divine patron and win, the fan base has reacted to this idea with little approval.
 * Perhaps a better idea would be to have the concepts they attuned to be one of the casualties of She Who Lives In Her Name's Three Spheres Cataclysm. These lost shards can only attune to people who embody, say, Flarg, but since no one can Flarg, these Shards are sadly not able to Exalt their chosen.
 * On the Terrestrial side, while canonical Dragon-Blooded can only be aspected to the five Gaian elements (the big four, plus wood), it's a common hobby to try to make Dragons of the five Underworld elements (ash, bone, pyre flame, blood and void]] or the six Autochthonian elements (crystal, lightning, metal, steam, oil, smoke).
 * Since every Splat in the New World of Darkness can have more-specialized splats (Bloodlines in Vampire: The Requiem, Lodges in Werewolf: The Forsaken, Legacies in Mage: The Awakening, etc.), it's painfully easy to crank out a new collection of Bloodlines, Legacies, etc., every couple of months. One book was even a collection of fan-created bloodlines.

Toys

 * Bionicle has plenty of humanoid species where only one or two named characters exist, and in order to expand on the main Toa cast all you need to do is choose any of the Elemental Powers, add a Mask Power and a weapon with optional attribute. Or, you could try your hand with the numerous Dark Hunters, which are already a mixed bag of various species, powers and motives.
 * The same applies for pretty much everybody in the Bionicle canon.
 * For Hero Factory, the successor line to Bionicle, LEGO created the Hero Recon Team. In-universe, the Recon Team is an intelligence force keeping tabs on the locations of villains. Out-of-universe, it's a program that lets fans create their own in-canon original characters and buy toy versions of them.
 * Sadly, you can no longer purchase the figures. It is now simply a watered down creation tool, like the designer software.

Video Games

 * This isn't too hard for the Ace Attorney series. You just need to think of a Punny Name for your new lawyer/police detective/murder victim/suspect/etc.
 * Glaring personality quirks. Don't forget about the glaring personality quirks. In fact, if you make your character 100% normal, then he/she will be instantly shot down-- unless being 100% normal is the glaring personality quirk, since absolutely no one is "normal".
 * Countless numbers of new Freedom Fighters in Sonic the Hedgehog fanfiction. This is easier in fanfic concerning the Archie comic series, as there are any number of unnamed Freedom Fighter groups to work with. The issue comes when people shoehorn their new character into the existing groups, or if they somehow solve everything...
 * Back when Knuckles was said to be the Last of His Kind, multiple Mary Sue echidnas would show up with the explanation that she thought she was the last of her kind, or she came from another dimension, and so forth. This has pretty much stopped now that Knuckles really isn't the last of his kind anymore in most of the various canon universes.
 * Boy, when they played
 * Heck, many Sonic fan characters have nothing to do with the series at all.
 * In Mega Man X fanworks, new characters are most often created as Maverick Hunters, like the canon protagonists. And since X is part of the 17th Unit, that frees up at least 16 other units of indeterminate size for fan usage.
 * There's also characters who are either the 'real' last creations of Light or Wily, or prototypes of X or Zero, and were also sealed away in a capsule until discovered, and the most Sue-ish of which tend to have caused the "Cataclysm," the by-now Jossed Fanon theory that got rid of the Classic series characters.
 * The original Mega Man series easily allows for new Robot Masters or other new robot characters, who have musical pun names. The Mega Man X series simply uses animal names to get the same result.
 * In a similar vein, new scientists tend to come up, thanks to the series having so few of them (Drs. Light, Wily and Cossack). Occasionally, some of them do follow Cossack's musical naming theme.
 * It's also popular in the series to do this with Protoman, whose infrequent appearances in the canon mean there's plenty of spaces for fanfic writers to fill in for him.
 * Since X and Zero are the leaders of the 17th and 00th units respectively, it's also common to create new characters to make up these units and show them functioning as teams. And ever since Mega Man X Command Mission showed that the Maverick Hunters have multiple headquarters, like the East-Asia HQ shown in the game, opens up even more opportunities for other units.
 * Battle Network isn't hard to do this to either, being as there are several Classic Robot Masters and other robots that went unused by the end of the series, not to mention 6 bringing in Colonel and Iris and opening up possibilities for X series-based Expies. That's not even counting the possibility of random officials and/or other Netbattlers and their Navis made up wholesale, or giving an operator to Bass.
 * Organization XIII, from the Kingdom Hearts series, often acquires a 14th member in fanfiction, to be paired up with one of the other members. These fanfics probably contributed to the annoyance of much of the Kingdom Hearts fandom when it was announced that there actually would be a 14th member in a later game. The only real challenge to this is if the character sticks to the naming scheme, which is an anagram of the Nobody's real name with an X thrown in.
 * The canon '14th member' is an interesting case: she's  That has got to be a Take That on behalf of the developers.
 * Additionally, new keyblades get handed out like lolipops in the KH fanfic 'verse, so Sora and Company start running into new worlds and new Keyblade masters with such alarming frequency that you have to wonder how the original worlds were even lost in the first place. However, the series states that once there were thousands of Keyblade wielders, and more do keep popping up, so there's precedence for this.
 * Some Fanfics of Kingdom Hearts may actually show an entirely new Organization surfacing from all that was ever left of the old one. Considering the "Twenty Years later, a new disaster causes a group of people to lose their hearts and create nobodies"-situation is, in theory, highly possible....
 * Chances are you'll find new types of Heartless and Nobodies in fanfics. Given that most of the Nobodies are based on a Final Fantasy job class in name and abilities, creating new types is fairly simple. Besides, it is strongly suggested that there were five more classes of Nobodies that were never seen. Ten types of Nobodies were in Kingdom Hearts II: two common types (Dusks and Creepers) and one type of minion for each of the eight Organization members still alive at the beginning of the game. There is no logical reason why the five who were killed at Castle Oblivion wouldn't have had their own minions.
 * Ratchet and Clank is so loosely written in canon with any number of worlds and characters that he can encounter in any given game that it's a given that you'll have to invent a couple characters. But any OC that pops up as a Lombax is an automatic Sailor Earth! Strangely, the majority of Sues in the fandom are either humans or humans-turned-Lombaxes, so...
 * In Fate/stay night by Type Moon, the First and Second Grail Wars have been COMPLETELY blank for Masters and Servants, as well as only 2 Servants named and 2 more mentioned for the Third Grail War with 0 Masters. Some fan stories have filled in their events, commonly stuffing the first one with purely Japanese spirits. Some even add in new classes based on a few throwaway lines.
 * Tsukihime has a sort of most-wanted list of bad guys, the 27 Ancestors of the Dead Apostles. Several of these are still unrevealed, making it easy to introduce a new villain as one of them.
 * As noted in General, it's trivially easy to add a "baddest of the Badass" new ship in any game or series based in space, particularly when there's such a potentially huge war zone where only relatively small slivers of it have been revealed, like in the Wing Commander or Star Wars game universes.
 * While there were about 30 Spartan-IIs in the Halo universe, less than half of them have been named so far. This makes creating a new Spartan character as easy as picking a first name and a number. Just don't expect your Spartan to make it past the fall of Reach -- every survivor of that battle is canonically accounted for.
 * There is however the Spartan-IIIs, hundreds of which were produced, a handful of which survive their first and only major deployment, whom almost no one knows about due to an extensive ONI coverup. It could be argued that Ghosts Of Onyx was written with the intention of expanding the unfilled roster to allow more Spartan-centric stories to be written.
 * And as it turns out, the fact that there was only one class of Spartan-I Is has since been retcon'd away, so people can make Sailor Earths of them all they want (one of the team mates in Halo Reach is from the second class of Spartan-I Is).
 * In the Touhou canon, youkai apparently outnumber humans by a fair margin. If the sheer diversity of those in Japanese mythology isn't enough fodder for the fans, races like the Celestials, the Yama and the Kappa only have one representative each, allowing fans free rein to make their own. Of course, the franchise has the unique distinction of being largely Fanon anyway.
 * Why limit Touhou OCs to Japanese mythology? Canonically, the vampires, psychically-produced poltergeists, resident Cute Witch, and fairies all have strong Western influences, and there may or may not be a Chinese dragon in the mix as well.
 * This isn't even limited to youkai. Aside from Keine and Akyu none of the residents of the Human Village are known (and those two aren't exactly human anyway), and it's canon that humans from the Outside find themselves in Gensoukyou on a semi-regular basis. The latter in particular is incredibly fertile ground for Self Insert Fics.
 * Persona fanfics will probably involve someone else learning to use a Persona. Or another member of SEES. Or an employee of SEBEC. Or another member of the Masked Circle or who survived the fight in the TV.
 * The Legend of Zelda-fanfic writers love this trope. Remember the Urban Legend of Zelda that there's actually a fourth Triforce, which balances and completes the other three? This (very unlikely) theory is often taken as premise for fanfiction and along comes a fourth golden goddess and a fourth triforce-bearer, usually female, So Beautiful It's a Curse and related to Link in some way. Or the fourth triforce goes to an Author Avatar, who was "sucked into the game", another very popular premise in the world of Zelda-fanfiction.
 * Three words: My Inner Life.
 * There's also "Kasuto" in Ocarina of Time. To elaborate, every town in Zelda II had a character in Ocarina named after it, with the exception of Kasuto. This has occasionally overlapped with the fourth Triforce bit.
 * On the subject of the fourth goddess bit, it turns out in Skyward Sword that
 * Team Fortress OCs tend to come in the flavor of the (currently nonexistent) "tenth class" or as the First Female [Class], and most of the writers don't bother to address the Values Dissonance that most, if not all, of their prospective teammates would have 1960s sensibilities.
 * Of course, there's the other idea with regard to Team Fortress 2 OCs in that the classes are used as templates and original characters are laid over them. This is seen in Cuanta Vida, where there can be two class members with entirely different personalities.
 * This is very common in Left 4 Dead fanfic, as many writers create a new Special Infected which makes the Tank look like a Muscular Chipmunk, or adding a 'survivor' who cought the Green Flu only to gain all the superpowers and none of the flesh-hungering side effects.
 * Super Mario Bros is subject to this, due to how little several settings or groups are featured or elaborated upon, mostly Bowser's troops.
 * Or Sarsaland. The Mushroom King. Beanbean Kingdom. The Shroomers. More plumbers. Worlds 1 through 8. The Yoshi's. Subcon. It's really too easy. The world building is there, we're just not given all of it.
 * Speaking of Mario, it's not very uncommon (or hard) for someone to create a whole team of Paper Mario partners. Simply dress up some of the series signiture mooks (Goombas, Koopas, Boos, maybe throw in a Toad or Yoshi or original creature in there), make sure they have the ability to explode, jump gaps, disappear, etc., and a new team is set.
 * It is given some elaboration in Yoshis Island DS, which states that there are a total of seven people as powerful as Mario and Luigi, and they're all named. That doesn't stop anyone.
 * It's easy to add pretty much anyone into the Silent Hill Universe without major canon issues, so you can expect that a lot of those people are going to be self-insert/wish fulfillment characters, or outright angsty Mary Sues.
 * Very easy to do in Dragon Age Origins. Despite that the box says the main character is among the last Gray Wardens, Alistair mentions this is only true in Ferelden, the game's setting. Speaking to him brings up the mention that there are gray wardens all over the world and in neighbouring countries - a Sailor Earth could simply be from another part of the world. Ridiculously easy to do for an Elsewhere Fic.
 * Indeed, the expansion pack, Awakening, uses this very setup - if you choose to start a new character rather than import the hero from Origins, they will be a senior Warden from Orlais sent to help replenish the ranks of the Grey Wardens in Ferelden.
 * In Borderlands, a Sailor Earth could simply be another person like the four playable characters. Or if you wanted to get in terms of powers, one of the 5-6 other "Sirens" like Lilith.
 * Metal Gear Solid has the three Snakes: Solid, Liquid, and Solidus. The absence of the most obvious third entry in the matter-based naming cycle, Gas Snake, makes him an easy choice for this. Plasma Snake would be a somewhat close second. However, Solidus Snake fills the theme roll of Gas Snake. Bose-Einsteinium Condensate Snake, however, has not had much support.
 * The general concept of Council Spectres in Mass Effect alone is ripe for this trope. Outside of Shepard, Nihlus, Saren and Tela Vasir (all of which have been on screen), the Spectres in general have been barely given any attention in game or in the Expanded Universe, with the exception of Blasto, the first Hanar Spectre.
 * Fan characters in both Spyro the Dragon canons are popular, ranging from simply other dragons to another survivor of Spyro and Cynder's Year of the Dragon. Other popular topics are a Wind Guardian or a replacement for, as well as Purple Dragons between Malefor and Spyro.
 * Dissidia Final Fantasy is an Intercontinuity Crossover bringing together thirty Final Fantasy characters from all over the series to fight on behalf of the gods Cosmos and Chaos. Dissidia and its prequel Dissidia 012 explain these wars occur as a cycle--when one side loses they get their memories of the battle erased, and are revived to fight again. The two games cover the 12th and 13th cycles, and the 13th one of the last. That leaves Cycles 1-11 unseen, with no real information on who was fighting in them, for which side, and what happened that they didn't make it to future cycles. As a result it's common for fanfiction to be set in one of those earlier cycles with new characters drawn into the war, and from there you can take the story pretty much anywhere you want it to go.
 * A very easy way to make a Mortal Kombat fan kharakter would be to take a kolor that hasn't been used by a ninja yet, and make your very own Highly-Visible Ninja for that kolor.
 * Apparently, New varieties of hellspawn from Doomworld, as well as loads of custom-made megawads. In the case of the former, though (possible via Dehacked or by using features of a certain few sourceports.), it is possible to create recolored enemies, completely different versions (In most cases, either more annoying or far more lethal) versions of weaker enemies such as the Zombieman. Certain monsters, such as the Cacodemon or any of the UDoom endboss enemies have quite a few variants of them, to say the least. However, there are also a handful of actual original doom monsters out there.
 * Apparently, New varieties of hellspawn from Doomworld, as well as loads of custom-made megawads. In the case of the former, though (possible via Dehacked or by using features of a certain few sourceports.), it is possible to create recolored enemies, completely different versions (In most cases, either more annoying or far more lethal) versions of weaker enemies such as the Zombieman. Certain monsters, such as the Cacodemon or any of the UDoom endboss enemies have quite a few variants of them, to say the least. However, there are also a handful of actual original doom monsters out there.

Web Comics

 * As one might guess from the title, The Sailor Sun webcomic makes full use of this trope even though it should not be confused with the trope itself.
 * Sailor Ranko, another webcomic based on Sailor Moon fanfiction, in this case a well-known Fuku Fic series that uses the Sailor Earth trope to shoehorn Ranma One Half characters into Sailor Moon.
 * Prematurely shot dead by Periodus, after the creator claimed that now someone could make Mary Sue fic about Oxygen, and a commenter threw out a several-sentence parody.
 * Homestuck revolves around sessions of a special computer game called Sburb. It's not uncommon for fans to invent their own characters who order and receive the beta in the mail, particularly on the Forum Adventures section of the forums.
 * It's implied that the four canonical kids are the only ones left, given that there seem not to be more "exiles" and the kids' meteors are by far the biggest ones (presumably marking them for the only session that actually gets in). It's implied that nobody else who actually received the game actually got into the medium because they couldn't figure out how to get into the Medium, which is portrayed in canon as a fairly simple process. This of course is totally ignored by fan-adventure writers.
 * It's only Dave and Jade's meteors that dwarf all the others; Rose's twin meteors are pretty average-sized, and John's isn't given the same examination but is probably even smaller. The kids' Sburb session breaks a lot of the rules anyways, so having unusually gigantic meteors for half its players could be part of that anomalousness. Also, given that they have the entire planet Earth to be exiles in, it's not that implausible that the groups of exiles corresponding to the main and the Sailor Earth sessions just haven't crossed paths. (Given how much predestination figures in the plot, it could be quite reasonably Hand Waved as the exiles for each session being drawn to each other and only each other.)
 * There's no excuse, however, for the multitude of fan troll sessions - the twelve canon trolls are explicitly the only ones who will ever play Sburb (or rather Sgrub) because their version was coded by a few of their own players, not released by a company like on Earth. The most canon-friendly fan sessions are the ones that invent entirely new species for the players to be, which are in themselves an example of this trope given how similar they tend to be to the trolls - most fan-created species have their own romance system, calendar, family structure, and equivalent of bedding because the trolls were different in these fields.
 * Also, why does everyone always forget fedoraFreak? Text messages on Dad's phone reveal that one of his acquaintances (actually their neighbor) actually entered and survived in another instance of the Medium and presumably reached God Tier, since the last correspondence with him has him dying on a Quest Bed. That certainly provides a whole load of precedent.
 * Such a plot was actually used as an Alternate Reality Game-like explanation for upgrading the fandom's forums fora software.
 * A popular subset is making a Crossover - characters from another work play Sburb while chatting between each other. Sometimes, the author deliberately doesn't reveal what is the crossed-over verse, challenging the reader to guess what it is from the characters' chumhandles and conversations.
 * Related to this trope, the author has joked that every fan troll is canon. Even the bad ones.
 * Though it overlaps with Epileptic Tree, the hypothetical (well, formerly hypothetical) thirteenth Troll falls into this. Note that the actual thirteenth troll turned out to be an in-universe Ascended Fangirl, and is practically an avatar for Homestuck fandom. In other words...a fan troll.
 * A webcomic known as "Bishoujo Senshi Sailor Earth" exists, using an already existing character (Naru) as the titular Sailor Earth. However, she was not a part of the original Sailor Senshi team/group of friends and is added based on an answer the creator gave to fans about using Naru as Sailor Earth should she ever be needed.
 * Because Drowtales has an on-site RP section this is very common, though there are very specific rules about how created characters can be related to canon characters, and a few characters have actually shown up in the main story as cameos.

Western Animation

 * Transformers makes this even easier, even if you don't take a toy that never got a chance at the spotlight and use it as a character. Pick an alt for your Original Character and a name that fits the "Code Name as real name" motif, and you're pretty much good to go. You don't even need to say why they're with the other characters, as it's implied they're there for the same reason everyone else is. This is probably because Hasbro uses the same method for many toy-only characters.
 * And not-toy-only characters. In Transformers Generation 1, If you weren't a Combining Mecha team, thus requiring special focus for your intro because those are expensive (and cool) toys, then Remember the New Guy? was the only introduction you were going to get. If Omega Supreme can get the "was always here, you just never saw me!" treatment, your sportscar-bot getting it is no worse.
 * In addition to bringing in toys who never made it into the screen, there's also the fact that the franchise loves Reboots and not all the Loads and Loads of Characters are in every version. Rectifying this is highly popular. Animated Sideswipe? Movie Prowl? Why not?
 * Invader Zim is prone to acquiring new Invaders. Or Tallest, if you're feeling ambitious. The creator explicitly mentioned this on the DVD Commentary, saying he would enjoy going to the homes of the little kids who believe they're "Invader Tom" or "Invader Sally" and wrecking their dreams by noting they weren't at the Great Assigning (every Invader, even those that didn't appear, are accounted for).
 * Note that these new Invaders, or any other Irken OCs, almost inevitably wind up going to Earth and ending up in Ms. Bitters' class. Which seems awfully familiar to anyone who watched the "Tak" episode.
 * The same setup as Ojamajo Doremi goes for The Backyardigans, but it's for the purpose of increasing the cast. Also, let's not forget that the Backyardigans have the same theme colors as the five main Ojamajos.
 * With Stitch from Lilo and Stitch being Experiment 626, fans have approximately 625 remaining experiments to come up with for themselves. The TV show however, showed some of these experiments and the finale movie listed all 625 of the other experiments in the credits. There's always 628 and up, though. Especially 628, who is very conspicuously shown as a pod, and only as a pod, in one episode's ending.
 * The 628 pod was retconned away, as Leroy was labeled as 628. That episode did show, however, that Jumba has a way of not learning his lesson, so 629+ are still fair game.
 * Foster's Home for Imaginary Friends Fanfic tend to introduce new imaginary Friends moving into the home. However, they're frequently Mary Sues. And move into the same room as Bloo. It's even worse when the author decides for some weird reason, that Bloo should fall in love with this Sailor Earth Mary Sue. Bloo, who never ever showed any affection for female creatures, except for Frankie. That was once, when he saw her in a dress for the first time, and his affection was limited to chanting "She's hot!" all over again.
 * Since there are only three official Powerpuff Girls around, in came a rainstorm of PPG OCs, mostly being Powerpuffs for any other color besides pink, blue and green. When it's not the rest of the rainbow, it's just a new-and-improved Bunny brought Back From the Dead and beautified. One of the most well-known Powerpuff Sailor Earths around is Bell, the white Powerpuff from the Powerpuff Girls Doujinshi by Bleedman. Even then, she's an antagonist.
 * Theoretically is is possible of someone else gets their hands on Chemical X. The Girls themselves tried it with Bunny. And then we've been shown that living things can be given super powers with chemical X in the form of Bullet.
 * It's also been proven that to make a construct like the girls, you need proper measurements, or you get Bunny or the Xtreme Powerpuff Girls. In addition, Chemical X has a temporary effect on already living things if it's not given in proper doses. Besides, it's far more tiring to see the RRB's brought back nice than the new girls which have been possible in canon and quite entertaining in their variety.
 * The sheer amount of characters in the later seasons of Justice League Unlimited make it easy for anyone and everyone to enter their OC into the League with absolutely no disruption of canon at all, except for the distortions that Sues and Stus cause if their new characters happen to be them.
 * Shoehorning new Lyoko Warriors into the group in Code Lyoko fic is all too common, as they are in a whole school full of potential new warriors, and they have endless need for new help. And heck, if your character is a girl, she's the perfect thing to fill in Odd's empty love interest slot. The only problem with that is, the one time in canon that the Lyoko Warriors tried to bring a new teammate on board, it didn't work out too well for them.
 * Teen Titans fandom contains PLENTY of fics where a new girl or boy (far more rare) shows up in Jump City with uberspeshul powers and a tragic past, and joins up as the sixth Titan. This character is often a Copy Cat Sue of Raven or Terra. And most are set well after Season 2, where Terra did it and sold them out to Slade, making it ridiculous for them to just add one on quickly out of nowhere. The BEST they'd get is honorary status (a communicator and an offer of occasional help in an emergency), keeping them out of the tower to prevent another Terra from sabotaging them from within.
 * Plenty of these new Titans seem to indeed be Terra clones, in that they're either working for or related directly to Slade. OC apprentices were dime a dozen for quite a while.
 * The series ends on a very open-ended note, leaving that opportunity wide to the masses to make something of the sort.
 * Avatar: The Last Airbender finishes on something of a cliffer. Though this is said to be expanded upon later in Avatar: The Legend of Korra, there are many fics that attempt to write a Book 4: Air (which would make sense, considering how much the show focused on the four elements), creating a number of copy cats of any number of characters, be they of the water tribe, Toph's family, relatives of Fire Nation elite, or, obviously, Air Nomads.
 * There are tales about a previous Avatar: Pretty easy to do, since we don't know how may Avatars there have been and we only know the names of six of them (Aang, Korra and the four Avatars immediately preceding Aang: Roku, Kyoshi, Kuruk and Yangchen).
 * Another variation is a story about the complete learning cycle of a later Avatar, anywhere from directly after Aang or Korra to an Avatar in a modernized setting. Very rarely it inverts the concepts and has an Avatar living in a futuristic space age.
 * Recess always has the possibility for "The New Kid", or any other of the nameless kids (or their nicknames) present on the playground. It wouldn't be that hard for something like "The Slide Climbing kid" or "The Video Game Kid".
 * Winx Club allows the creation of infinite Sailor Earths, as each planet of the Magic Dimension can have as many Guardian Fairies as needed, not counting the Witches nor the Specialists. And even when the show has its own share of fanfics, its visual and fashion-centered approach prompts the fandom to create new fairies with their own outfits and their own transformations. And yes, there is such a thing as Sailor Enchantix and Enchantix Moon.
 * Every version of Strawberry Shortcake also allows the creation of infinite Sailor Earths. The characters are named for food. Limitless, eh?
 * It's not hard to have OCs for Codename: Kids Next Door. Just pick a number.
 * Or become a member of the Safety Patrol. Or be delightfullized. Be Crazy Awesome as a child or teenage character. Make some sort of villain (just come up with a concept you dreaded as a kid).
 * Total Drama Island, by having its characters based on teen stereotypes, has many fanfics where either a) two new characters (often two girls) join the cast as part of the first season, or b) starting a whole new season by making totally original stereotypes that have oddly very similar characteristics...
 * More audacious fics such as Total Drama Battlegrounds tend to introduce a whole new cast of stereotypes while keeping the original cast as well.
 * Fans tend to create their own 9|stitchpunks with numbers of 10 and higher. Which may or may not totally defeat their original purpose, or maybe the fans just imagine themselves to be the scientist's partners who all decided to band together and create an army of stitchpunks for a more likely chance of destroying the machine, or... whatever, my brain hurts.
 * Xiaolin Showdown gets this a lot too, though the fanfics seem to be split evenly between making a new kid show up, or making Jack a Xiaolin monk as well. As with the Digimon Frontier example above, this always involves making a "new element". The most common choices are metal, plants/flowers, and on occasion, Heart.
 * Many Road Rovers fans loved creating their own, by simply picking a breed, gender, nationality, and maybe superpower.
 * Another fandom that makes this easy is the Storm Hawks fandom. All you need is to either pick a pre-existing species (Merbs are the most common in fanfiction) and pick the name of a bird. Most of these also tend to be a love interest for Stork, of all people...
 * TUGS revolves around two fleets of tugs. To introduce a new character, all that's needed is for one of the fleet's captains to suggest that they need another tug because there's too much work, or (in the case of the Z-Stacks) to make them more evenly matched against the Star Tugs. The series also suggests there are lots of other tugs, and there are plenty of locations in and around the port that we haven't seen yet. And then there's the many background characters that were never developed during the series run.
 * Considering the masses of characters in the Legion Of Super-heroes, slipping in a hitherto-unseen OC who recently got transferred to the main team(aka, the relevant team) is easily done.
 * Because of how common it is for X-Men: Evolution fans to create Original Characters, some people have advertised stories where the basic premise is 'Create your own OC for my new OC group of Students'. It's a review gold mine on Fanfiction.net.
 * Similar to Transformers, you can always bring in your favorite comics X-Man, Legionnaire, or other who wasn't in the show. Some of these take on Possession Sue status.
 * My Little Pony has Loads and Loads of Characters in pretty much all its incarnations, and there's a ton of "OC Pony" fan-art out there. My Little Pony Friendship Is Magic has inspired fanfic not only about the "Seventh Element of Harmony", but Evil Counterparts to the Elements (Generosity vs. Selfishness/Greed, Laughter vs. Despair, etc.), usually known collectively as the Elements of Discord.
 * Trixie making a Heel Face Turn and becoming the Seventh Element is quite common.
 * Fan favorite Derpy Hooves is often depicted as hanging out with and interacting with the Mane 6. Her fanon-established daughter Dinky is similarily often included with the Cutie Mark Crusaders (despite having been repeatedly depicted with a cutie mark, although it keeps changing).
 * Besides, as it is a world inhabited by what's deemed to be MANY Ponies, one simply needs a name and a cutie mark... you would not be hard pressed to find a brony with a Pony OC.