Fanfic Fuel

"Johnny Yong Bosch: Maybe kids will write some fanfics about it. Linkara: We can only hope."

Many times a creative work will leave large gaps in a character's Backstory, or not mention what happened between two instances of seeing a character, or object or place. We might get a bit of explanation, usually a throwaway line that suggests a Noodle Incident, but either way we are left to imagine it ourselves.

So, someone else will, of course, do it for us. Many fanfics that don't involve Shipping will fill in any gaps, often with wild adventures that one would expect would get more of a mention in the work if they'd happened, but don't. Anything in a work of fiction that leaves inviting gaps can be seen as fanfic fuel. The more outrageous or open for interesting plot developments, the better.

Summers between school years in high school dramas are often great sources of Fanfic Fuel, as well as the draw of giving a mysterious character more backstory (which may result in Draco in Leather Pants) or rewriting things from the perspective of someone else whose viewpoint we never saw in canon.

See Canon Fodder, a subtrope.

General

 * Many of the "could-have-beens" on the What Could Have Been page are prime fanfic fuel. The same goes for the ideas on the Wild Mass Guessing pages.
 * Similarly, many one-shot comics and manga (as well as tv pilots) can be used as plots for fanfiction.
 * And the Adventure Continues... invites possible sequels.
 * Any work with a Time Skip that doesn't cover the intervening period in enough detail.
 * Putting two characters in a cave or similar small space together will result in a few hundred fics about them getting it on. Please remember that Most Fanfic Writers Are Girls.
 * Sequel Non Entity characters are practically engraved invitations. Something must have happened to them to prevent their appearance in the sequel.

Anime and Manga

 * Infinite Stratos, nuff said. For average viewer, it's just a mediocre Unwanted Harem WITH MECHA. Fan Fic writers on the other hand... War, politic, social shift... the potential is limitless, and three point above is rarely addressed in the canon source (except small clues here and there) despite it was said the introduction of titular Powered Armor more than few things changed, leaving the gap for the fic writers to explore. It was not rare to find the sheer difference of atmosphere in I.S. fanfic as the result, from Romantic Comedy / Shipping to Grimdark War Fic, despite the tiny size of the fanbase.
 * Death Note has lots of rules that don't come up in the main plot. Example: if someone's name is written in two Death Notes within a split second of each other, it is regarded as simultaneous and the victim lives, unaffected by either note.
 * Backstory: L's / Mello's / Near's / B's / Matt's Dark And Troubled Mysterious Past. What does Wammy's House do and why was it built? / Light's career as a teen detective and/or what would happen if Light didn't find the notebook or what would happen if
 * Dragon Ball has lots of these. Like the five years between Dragon Ball and Dragon Ball Z, the time between the Cell and Majin Buu arcs, the years that Goku spent training with Kami-sama, the years of training before the attack of the Androids 19 and 20... there are many others like those. Also, there is the fact that we know there were at least 20 androids created by Dr. Gero, but we don't know all of them; the fact that Saiyans were sent to other planets when they were babies to grow up with the planet's civilization to later destroy them and come back to Planet Vegeta, that let open the idea that there may be other Saiyans still alive, living in the planets they were assigned to destroy; the alternative Universe Future!Trunks came from, and the idea that more alternative dimensions can be created with the Time Machine... Dragon Ball is totally made out of fanfic fuel. But it was to be expected from a story of such length (and that was mostly improvised by Toriyama!) and for all the Plot Holes created by the anime series that needed to keep adding fillers while waiting for the manga to catch up.
 * It doesn't help either that almost all the movies could be considered Alternate Universes because they don't fit very well with the main continuity, so you have many possible universes to tell stories about.
 * Cowboy Bebop persistently reveals a lot of background detail to the audience without actually telling them anything, leaving enormous room for speculation. Most of this concerns the past relationship between Spike, Julia and Vicious, though other characters (especially Ein, Wen, and Mad Pierott) aren't exactly lacking.
 * Fans of Axis Powers Hetalia have plenty of fun writing for the nations, with all the history between them.
 * The Naruto fandom already were fond of writing Alternate Universe fanfiction set in high school but when the anime released an ending theme that pretty much set up the roles of each character for them the amount of fanfiction (and fanart for that matter) exploded.
 * Sailor Moon spans millenia and follows only a dozen or so girls of what could be hundreds. Naoko did say that there could be senshi all over the universe, and we even meet a couple dozen alien senshi in the last season. Therefore, there's reams of Silver Millennium, Crystal Tokyo, and "Team Other Than The Sol Senshi" stories out there, and the pile grows all the time.
 * Three words: Ash Ketchum's father.
 * Zero no Tsukaima starts with Louise summoning a familiar, and accidentally summoning a human from another world. Every single fanfic starts by simply changing who she summoned, usually as a crossover with another 'verse.

Comic Books

 * The X-Men, and all related books, went through a canonical "six-month gap" around 2000, where Cyclops was believed dead and Cable joined the team to take his place. All that is known at the end of it was that Psylocke and Phoenix had switched powers, giving the former telekinesis but no telepathy and the latter, the reverse.
 * In Earth-3490: Captain America (comics) married Iron Woman and thereby prevented the Civil War. So how did Natasha Stark become Iron Woman? How did this affect team dynamics? What was the wedding night like?
 * Any comic book universe with alternate timelines throws the door wide open for any number of AU fics.

Film

 * Happens a lot with The Dark Knight Trilogy fanfics featuring how The Joker got those scars of his, simply because he was so inconsistent about them in the film.
 * An example from Transformers: Dark of the Moon. By the third film, The Masquerade is completely broken, and everyone is aware of the Autobots' existence. The movie doesn't go into much detail about this, but there are many ideas that can inspire a fanfic. One is how the nations of Earth are shown to dislike the Transformers (there's a mentioning of poll results stating that half the world would feel safer if the Autobots left). One must also wonder how other countries feel about the US using Autobots for non-Decepticon related problems (such as destroying illegal nuclear plants in the Middle East). Overall, there's a whole potential to explore how Earth must cope with the knowledge that one of the world's most powerful nations has Humongous Mecha on its side, as well as the fact that said support also warrants even more alien attacks.
 * There's also the matter of why Mikaela broke up with Sam.
 * Before Dot M, there was a fairly large number of Day in the Life-style fics regarding Ironhide and Captain Lennox's family.

Literature

 * The Harry Potter series has Loads and Loads of Characters (students, ghosts, professors, shopkeepers) and several summer holidays for those characters to have adventures on. The main trio in Deathly Hallows are away from Hogwarts for much of the year, so that leaves even more gaps to fill.
 * Also, the very explicit, but non-detailed events and descriptions mentioned by various characters throughout the series, coupled with very effective characterization set up by the chapter "Snape's Worst Memory" have given rise to a huge quantity of fanfic set in the era of Moony, Wormtail, Padfoot and Prongs, with dozens of different plays on various unseen canon events.
 * And there's the short span in which Ron and Hermione are absent from the narrative in Deathly Hallows, during which they get basilisk fangs and destroy a Horcrux. Considering how emotionally traumatizing destroying a Horcrux is, and that they have their first on-screen kiss immediately following this, there's an entire sub-genre of fanfic there.
 * Every fan has their own opinion about the various inconsistencies of the saga. Some issues (like how the school(s) were created, how magic and technology have evolved together, or how British wizards can possibly be so bloody dumb) are almost never addressed and are calling for a lot of imagination.
 * Two points in The Chronicles of Narnia have launched a thousand fics:
 * 1.) Most of The Lion, the Witch and the Wardrobe covers the first several days the main characters spend in Narnia. C. S. Lewis devotes approximately two paragraphs in the final chapter to the following years—or likely, decades—they spend in Narnia.
 * 2.) Susan's absence in The Last Battle is brought up once, quickly dismissed, and never mentioned again. Adding to the potency of this fuel is the phrasing of the reason given for her absence.
 * The Hunger Games has Elsewhere Fic fuel aplenty. After all, someone (be it a canon character or an OC) has to have won the Games for years 1-73, and most of them we know little or nothing about.
 * Many times in the Sherlock Holmes canon, Dr. Watson includes references to cases that Holmes has solved but which were not fleshed into stories, such as the the tale of the giant rat of Sumatra (a story "for which the world is not yet ready"). A large percentage of Fanfic, including published pastiche novels, centers on one or another of these Noodle Incidents.
 * A Wrinkle in Time, in addition to all the science fiction elements not explored in great detail (the history of the planet Camazotz, the 2-dimensional planet, etc.), there's also the specific story of the project Meg's father was working on (with the tesseract) and what happened to the first man who tried to tesser. He's mentioned very briefly, when Meg's father is giving Calvin some of the backstory about the project. He left successfully, but they never found out what happened to him (and none of the sequels give any indicator, either).

Live Action TV

 * Given that it's a show about Time Travel, Doctor Who has lots. and the Doctor's time with Queen Elizabeth I are just the tip of the iceberg; pretty much anything that isn't shown onscreen or in an audiobook/audio drama is game. And sometimes even then.
 * Lady Christina de Souza's past and future capers, especially now that she has a flying bus, could qualify.
 * And of course, any adventures Jenny may go on to have.
 * Or for that matter, any adventures Vastra and the other Jenny have already had.
 * There is the alternate universe that was first visited in series two and where Mickey decided to stay to help the effort against the Cybermen; at the end of the series and in series four . What else could the fanfic writers want?
 * Not to mention various fan attempts of fleshing out companions into more rounded and complex characters than presented in the show (Tegan, Nyssa, Peri, Mel, and other underdeveloped companions get this treatment in fan fiction).
 * The Whedon Verse have seen a lot of these.
 * Buffy the Vampire Slayer:
 * Wish!Verse: Remember that Alternate Universe where Buffy never came to Sunnydale? How did Xander get turned? How did Willow? What happened with Darla? How did Angel became a wreck?
 * Insert!Dawn-Fic: Everyone in the Cast was given false memories of Buffy's little sister Dawn in the Fifth season. How would any given episode in the first four seasons play out with Dawn there?
 * Normal!Verse: "Normal Again" told us that the entire show was a hallucination being created by Buffy in an insane asylum. Describe that world. Maybe the Big Bads are all just manifestations of her doctors.
 * Yet Another Halloween Fic: Also called YAHF for short, these deal with the episode where people wearing cursed Halloween costumes started turning into the things they dressed as. Halloween World is probably the most famous among the fandom.
 * Hell, Halloween Fics have sprouted a tiny subgenre: Xander gets all the abilities and hardware (including a certain blue AI) of Spartan-117
 * Angel:
 * Connor!Be-Gone-Fic: Angel Deal with the Devil-ed his son into a happy family and removed all memories of his son from his crew. How did this affect their recollection of seasons three and four? How did Wesley get that scar? Also obvious fodder for Connor/Dawn angst-fics.
 * Firefly:
 * ... anything, really. How Wash and Zoe fell in love, Simon's time studying as a doctor, what Book did before he was a shepherd (heck, let's just say everyone's backstory), what was the first time Patience shot Mal, the War of Independence, and perhaps most importantly, where does the ship go next after ?
 * The alternate universe on The Middleman. What happens with Fatboy Industries? Do alt!Middleman and alt!Lacey save the world? And what does aerosolized soup taste like, anyway?
 * NCIS: Season gaps tend to inspire this on NCIS. How did Tony hold the team together after Gibbs quit? How did Gibbs adjust to his temporary team? How did Ziva survive Somalia? Look up "NCIS" and "Somalia" on fanfiction.net, and you get over two hundred results.
 * iCarly: Everyone's parents. Out of the 6 parents of the main trio (and Spencer, who is Carly's brother), only 1 was shown (Freddie's mom Mrs. Benson), 1 was referred to (Sam's mom) until she showed up in a single episode, Spencer and Carly's dad is in the military and rarely shows up in one-sided phone conversations. This leaves Sam's father, Freddie's father and Carly/Spencer's mother, and they have never even been mentioned.
 * Why did Freddie end up in Seattle, or more accurately, why did his mother come to Seattle on her own, with him. Often combined with the father question.
 * Similar with the parents of characters from sister show Victorious. Only Tori and Trina's parents are seen regularly, Jade's dad was mentioned on one occasion, and both Beck's parent's appeared off screen in on episode. Often fics that mention the parents will have Andre's parents dead, Jade's parents either abusive (especially if it's an angsty fic) or just neglectful and uncaring, the latter being more supported by canon. Cat's parents are generally pretty neglectful as well, though just how neglectful they are depends on the fic. Robbie, being the Chew Toy and The Woobie will almost never, ever, have a happy home life, and as of "Locked Up'' that view is pretty supported by canon. Beck's usually the only one who will have anything close to good parents, although there will usually be a few issues considering that he lives in an RV just so he can get away from them.
 * It's easy to wonder about all the possibilities for adventure surrounding the concept of the Red Pirates in Kaizoku Sentai Gokaiger, as they traveled the universe gathering the Ranger Keys.
 * Supernatural gives fans Dean and Sam's entire childhood to deal, Sam's "Stanford Era" is popular, though it doesn't necessarily focus on Sam. The rest of their childhood leaves plenty of questions. Just how much Parental Neglect did the boys actually suffer? How many schools did they go to? What caused the rift between Bobby and John? The list goes on.
 * There are also the multiple alternate realities established/suggested.
 * What Is and What Should Never Be 2x20
 * It's A Terrible Life 4x17
 * The End 5x04
 * My Heart Will Go On 6x17
 * Also, the four months Dean was in hell (or the 40 years, depending on the brother the fic focuses on)
 * Bones: the post-Season 5 hiatus(during which time Booth and the Jeffersonian team went their separate ways for nearly a year) has produced quite a few fics. Many of them are also Fix Fics, designed to get Booth and Brennan together, usually eliminating Hannah Burley in the process.
 * The fact that we didn't actually see Booth and Brennan finally resolve their Unresolved Sexual Tension in "The Hole in the Heart" has been fodder for more than a few fics. One notable(and lengthy, at nearly 200 chapters and still going) fic, "The When And The How: A Bone to Pick", suggests that they didn't actually do the deed while she was staying at his apartment after Broadsky killed Vincent, but sometime a few days later, prior to "The Change in the Game", after dealing with Vincent's death and getting some perspective on what they really mean to each other.
 * A surprising number of Lois and Clark fics were inspired by the episode "That Old Gang of Mine", specifically one scene where Clark is shot at point blank range by a mobster's bullet. More than a few stories tried to explain how Superman dealt with the possibility that he might never become Clark Kent again, since the rest of the world thought Clark had been killed.

Tabletop Games

 * Warhammer 40000, due to the focus on selling miniatures instead of telling stories and the preponderance of Unreliable Narrators, is pretty much this trope incarnate, with even official material presenting vastly contradictory interpretations of factions, characters and events, the fans following the same policy.

Theater

 * In the Wicked musical, the fact that but Glinda is Locked Out of the Loop is shipping fuel for Fix Fics and Dark Fics alike.
 * Not to mention the 2 to 5 year (depending on what you like to believe) gap between Act 1 and 2 where we go from Elphaba Defying Gravity to Fiyero and Glinda working for the Wizard and Nessa becoming Governor and enslaving Boq. There's a tonne of missed action just there. Also the fact we only get a few Shiz scenes in the musical...

Video Games

 * Touhou Project. Oh Yukari, Touhou Project. The lack of backstory for many of the characters or the events of the PC-98 games are prime fanfic (and fan comic and fan video) fuel.
 * All of those spartans that didn't get a story on the Halo games and in the Expanded Universe.
 * Silent Hill, being a premiere Mind Screw series told exclusively from the view of the player character, contains a wealth of information for fans to explore, especially concerning the nature of the eponymous Genius Loci and the cult that worships it.
 * Many Golden Sun fanfics focus on what Isaac's party was up to during the first two thirds of The Lost Age, before they joined forces with Felix's group.
 * Several fics based on The Legend of Zelda Twilight Princess focus on.
 * Pokémon has generated massive numbers of fanfics over the years, thanks to the sheer cultural presence in the childhoods of an entire generation. The games(especially the early ones) often had sparse characterization and backstories for many characters(including the protagonists), making alternate character interpretations easy for many writers. And we're not even getting into the related media...
 * Metal Gear Solid fandom is particularly fond of writing fics about Liquid's backstory (usually set during his time in the Middle East) or the day The Boss killed The Sorrow. In fact, these two plots are so popular in MGS fan fiction that almost every male MGS fan writer has taken on the former, and almost every female MGS fan writer has taken on the latter.
 * Very little is explained about Malefor's backstory in The Legend of Spyro: Dawn of the Dragon. Any fanfiction that's not Spyro/Cynder shipping is probably this. His "death" in the final game was also vague, leading to many fanfiction and fan comics.
 * Soulcalibur V. Gets barely over a year in development. The length of story mode is drastically shortened due to time constraints, with numerous plot elements mentioned in official bios prior to the game's release being ignored or glossed over left and right and only a Japanese-only artbook attempting to fill in the blanks of exactly what happened in the seventeen-year interim between SCIV and this game (heck, there aren't even character-specific Arcade endings to help shed light on things). You can bet your soul (pun intended) fanfic writers are scrambling at this very moment to answer the various questions that SCV couldn't bother to.

Web Animation

 * Red vs. Blue's mysterious Project Freelancer has inspired numerous fan fics based on it, the questionable experiments it performed off-screen, and its various agents that all existed before the series occurred.

Western Animation

 * Many have wondered what happened during the time skip between the last episode of the second season of Transformers Generation 1 and Transformers: The Movie.
 * Also, since Transformers Animated left quite a few questions unanswered, plenty of potential there. Especially with the Allspark Almanac Volume 2 coming out, which will contain unused Season 4 material.
 * The Show Within a Show in Bolt could spawn dozens of fan stories.
 * Many people wondered what happened between the timelines of Justice League Unlimited and Batman Beyond. Fifty years had passed and many of the Justice Leaguers are gone. They've also wondered about the circumstances which led to Warhawk, son of Hawkgirl and Green Lantern, being conceived and born.
 * Invader Zim contains a surprising amount of background detail upon which numerous fans can and have elaborated, ranging from an interstellar war to Irken reproduction.
 * Especially since the show was canceled just as it was developing a Myth Arc, and the creators have revealed their basic plans to fans.
 * Road Rovers has loads of things for fans to expand on, including the relationship between Parvo and the Groomer, adding new Road Rovers or creating their own groups, and elaborating on the backstories of the Road Rovers. It also helps that the show only ran one season.
 * The fate of Ursa in Avatar: The Last Airbender. The show left a few other minor plot threads hanging, but that's the one that really got everyone upset, and speculating.
 * And, you know, a century-long war, with plenty of rich, undeveloped ground for exciting battles and stuff. And one thousand Avatars in the history of the world, too.
 * Galaxy Rangers? Wolf Den and the Supertrooper Project account for a lot of stories. So do scenarios on how to rescue Eliza from the Fate Worse Than Death.
 * Long before there was ever a series to take up the idea, Lilo and Stitch writers had 624 blank spaces to fill in with fan characters, a vaugely described laboratory for them to be born in, and a universe for them to explore. They made sure to take advantage of this.
 * Fans of My Little Pony Friendship Is Magic have had a field day filling in the gaps left by the series: why are Celestia and Luna apparently the only winged unicorns in the series? Did anything interesting happen in the thousand years between Nightmare Moon's imprisonment and the pilot? And let's not get started on the speculation fans have had about whether Equestria is the whole planet, or just one kingdom in a world possibly full of other talking ungulates...
 * The show has confirmed the "one kingdom" theory.
 * Luna's 1-year Chuck Cunningham Syndrome.
 * Now we have Daring Do, an Adventurer Archaeologist who is the protagonist of a Book Series Within A Show; the contents of one of the books have been shown on-screen, we know the title of another, and we know that there are many other books in the series. Cue lots of fan-made "Daring Do and the [x]" installments.
 * In "Hearts and Hooves Day", the Cutie Mark Crusaders create a love potion, but they only use a very small amount and leave quite a bit in the pitcher. Cue Shipping Fics involving one pony (or more) accidentally drinking the potion and hilarity ensuing.
 * The introduction of Tartarus and time travel spells in "It's About Time" has given rise to many fanfics
 * Now that the second season finale has aired, it's provided plenty more potential fuel. The biggest example is the finale's Big Bad Queen Chrysalis, who  You tell me that's not a dozen fics just waiting to happen.
 * Then there's the question of just how Shining Armor and Candence fell in love (not to mention why Twilight had a Princess for a foalsitter).
 * The Lion King has a lot of this. For starters, the movie Leitmotif, the never ending Circle of Life, means that you can write entire sagas of the royal lion family. Then, there are lots of characters that were introduced but not developed in comics or books, like Ni, Tama, Tojo. Also there are many story gaps that fanfic writers love to fill, like the years of Scar's reign, or were the Outlanders from the second movie came from.