Non-Linear Sequel

"So any belief that Nintendo will make a sequel to Zelda II the Adventure of Link is deader than $#*&. They can't even make them follow consecutive order! We have a sequel to the original, a prequel to the original, a sequel to the prequel, a prequel to the prequel, and a sequel to the young Link of the prequel's prequel! WHAT THE *&%^!? "

- The Angry Video Game Nerd on The Legend of Zelda

Some video games, especially RPGs, do not have a strict sense of a linear Sequel. The game may -- may -- take place in the same continuity, but not necessarily in the same time period. Occasionally the only thing similar is the system of play or shared tropes and references.

This is because video games, uniquely among media, have another dimension by which installments in a series can be interrelated, other than characters, setting, plot, or tone; that dimension being gameplay mechanics. Of course, this also means you might get an installment that isn't really one at all. Plus, it has the benefit of helping to avoid Continuity Lock Out--with non-linear sequels, it doesn't really matter if you start with, say, An RPG Adventure 1 or An RPG Adventure 10.

This also avoids the logical conclusion of why the hero isn't at Level 255+ when he starts the next game.

Many series of this sort have Recurring Elements.

Compare and contrast to a Thematic Series, which is a series that follows themes as opposed to specific characters or settings.

Action-Adventure Games

 * The Legend of Zelda is a bit of a mess, as Fanon persists in trying to find some coherent continuity between the games. Word of God stated the Time Travel shenanigans in The Legend of Zelda Ocarina of Time actually split the timeline, giving twice the elbow room for Fan Wank-ery. Though thanks to Shigeru Miyamoto's occasional nonsensical statements, nobody's sure if Word of God knows what it's talking about at any given time anyways.
 * The situation becomes even more complicated, as Word of God has a tendency to contradict itself on this matter. The Legend of Zelda a Link To T He Past is before the original two games, then it's after; Ocarina of Time has gone in and out of being considered the Backstory to Link To The Past. About the only constant has been that Ocarina is the major series starting point, giving a definitive origin to Ganon, the series Big Bad. Though even that position is being usurped by The Legend of Zelda Skyward Sword, which is giving a definitive origin for the Master Sword (which was already in play during Ocarina).
 * The series timeline has been recently revealed in the book "Hyrule Historia", turns out the timeline was split in 3 branches in OOT.
 * The Legacy of Kain series plays around with the Timey-Wimey Ball and as a result only the first two, Blood Omen and Soul Reaver, are in normal order. Soul Reaver 2 picked up where Soul Reaver left off with the main characters travelling back in time to a couple decades before Blood Omen, then they travel forward to several centuries after Blood Omen, then back to centuries before Blood Omen. The fourth game, Blood Omen 2, takes place between Blood Omen and Soul Reaver in an altered timeline caused by the fifth game, Defiance. Defiance picks up right where Soul Reaver 2 left off and switches between the two heroes who are in different time periods, one is still in the time period centuries before Blood Omen and the other is exactly in the same timeframe as Blood Omen, the events of that game occuring unseen at the same time as the events of Defiance. If you followed all that, congratulations, you just mastered one of the most complex time travel plots ever known.
 * Shadow of the Colossus and Ico take place within the same universe, with Shadow of the Colossus occurring earlier in the timeline.
 * Or at least, Word of God says that that's his personal interpretation, and everyone is free to come up with a different one if they want.
 * Bubble Bobble makes no sense. You got three second-installments and two third-installments, and chronological orders and release orders don't match.
 * The sequel progression in Tomb Raider is linear (while each is self-contained, stuff like artifacts in Lara's mansion show the progression) until Chronicles, which is based around events at various times in Lara's life, and the position of some scenarios in the overall timeline is very hard to work out (not helped by the way some events seem to violate previous canon). The Crystal reboot makes things much more complicated; while the changes to backstory and canon suggest a Continuity Reboot the majority of the previous games are referenced at points in the new games, suggesting that they DO still exist in the new timeline, which makes it difficult to work out where Legend and Underworld fit in.
 * Of course, it is entirely possible that the Crystal Lara had several similar adventures to her Core counterpart. It can't be that hard to believe that the two Laras have followed similar paths in life; after all, they are both armed archeologists who hunt mystical treasures.
 * The third Ninja Gaiden game for the NES, Ninja Gaiden III: The Ancient Ship of Doom, is an interquel between the original NES Ninja Gaiden and Ninja Gaiden II: The Dark Sword of Chaos (since Ryu lost the Dragon Sword at the end of II, but still has it in III). Ninja Gaiden Shadow is a very distant prequel to the first NES game, while Ninja Gaiden for Xbox and its sequels (Dragon Sword and 2) are set before the NES series (and presumably after Ninja Gaiden Shadow).
 * The Metroid series isn't a particularly confusing chronology, but the release order of the more recent games still doesn't necessarily match the order in which they take place, so it still qualifies. The order, as far as this troper knows it, is: Metroid/Metroid Zero Mission > Metroid Prime > ...Prime Hunters > ...Prime 2: Echoes > ...Prime 3: Corruption > Metroid II: Return of Samus > Super Metroid > Metroid: Other M > Metroid Fusion, with Oddball in the Series page-image-provider Metroid Prime Pinball being somewhere tangentially related to the first Prime game.
 * The games in Nihon Falcom's Dragon Slayer series are largely unrelated to each other. While Dragon Slayer VIII: The Legend of Xanadu is a sequel to Dragon Slayer II Xanadu, and Dragon Slayer V: Sorcerian has tenuous connections to Dragon Slayer Jr.: Romancia, it's perhaps just as well that "Dragon Slayer" was stripped out of the titles of most of the localized versions and the later sequels and remakes.

Fighting Games

 * Every 2D Samurai Shodown has been a non-linear sequel since Samurai Shodown II. III and IV are set after the original game but before II, V is a prequel to the first game, and VI is a "dream match" game with everyone from the previous numbered entries. Oddly enough, the 3D games are all set after Samurai Shodown II, although the PS version of Warriors' Edge (which is a different game from the arcade version) takes place in the distant future of the other games.
 * The Street Fighter Alpha series (despite its Japanese title of Street Fighter Zero) is actually set after the original Street Fighter (and Final Fight) and before the Street Fighter II series. The more recent Street Fighter IV is an interquel between II and III.

First-Person Shooter

 * The Halo franchise, chronologically, goes: Halo Wars, Halo: Reach, Halo, Halo 2 / Halo 3: ODST, Halo 3. The core trilogy was released first, followed by Halo Wars, Halo 3: ODST, then Halo: Reach. The anime serial, Halo Legends is both before, during and after the games, due to the multiple story format.

Hack and Slash

 * In chronological order of the plotline, the Devil May Cry series goes as follows: Devil May Cry 3, Devil May Cry, Devil May Cry 4, then Devil May Cry 2.
 * It's worth noting that the chronology came out like that because Devil May Cry 2 was such a mess the devs were jumping around to avoid mentioning anything about that game.
 * And now there's DmC: Devil May Cry, which was originally another prequel (well, more like Continuity Reboot) set before 3 and is now set in a parallel universe.

Platformers

 * The Castlevania series is the reigning king of bouncing around in the timeline, though the games all take place in the same continuity. Games have been set as far back as the year 1094 and as far forward as 2036. Konami didn't even wait till the series left the NES before starting this habit -- Castlevania III was set more than 200 years before the first game.
 * Though the various Mega Man games are released roughly in chronological order within their own series, the Mega Man Zero series which takes place after the end of the X series & before Legends began & ended years after the last Legends game came out. The newer Mega Man ZX series takes place after Zero but still before Legends.
 * Sonic the Hedgehog fans have had more than a few arguments trying to sort this one out.
 * Depending which region's version you follow, Tails Adventure is the first game, then Sonic 1, or Tails Adventure takes place in the same chronology it was released.
 * Sonic CD was developed at the same time as Sonic 2. A piece of bonus artwork (Tails with a caption saying "See you next game"), suggests to some of the fandom that this means Sonic CD is before Sonic 2
 * The Reset Button ending of Sonic '06 officially removed it from the canon. However, the game says that Blaze the Cat is from the same future as Silver the Hedgehog. Sonic Rush and its sequel directly contradict this saying that Blaze is from Another Dimension.
 * Also thanks to the Timey-Wimey Ball nonsense, Silver's future is perpetually doomed by something entirely different, and Eggman Nega is somehow from both the Future and another Dimension.
 * Sonic Chronicles then takes place two years after the last game of the series. However its ending This doesn't sit well with the next game in the series, Sonic Unleashed.
 * Unless that's the reason Sonic is trashing an Eggman armada at the start of Unleashed.
 * In Sonic Battle, Shadow has his memory back, or at least remembers the events of Sonic Adventure 2. Sonic Heroes contradicts this, with Shadow having contracted amnesia thanks to his fall at the end of SA2 (and maybe some other factors) and he still doesn't have memory of SA2s events by the end of Shadow the Hedgehog, making Battles place in continuity very iffy.
 * Are you sure? IIRC, he was pretty well-informed by the end of Shadow the Hedgehog.
 * Battle occurs after Shadow (as Shadow clearly remembers his past) but was released slightly before Heroes. Where it falls into the overall timeline is unknown, but it is referenced heavily in Chronicles.
 * Really, when it comes to the Sonic canon, everything is canon, though every game is in Negative Continuity unless another game feels like providing a sequel to a story.
 * Super Mario World 2 Yoshis Island, although numbered, is a prequel to the main Super Mario Bros. games.
 * Though it could be argued that since it's simply called Super Mario: Yoshi's Island in Japan, it was never meant to be connected to Super Mario World.
 * Wonder Boy III Monster Lair and Wonder Boy III the Dragons Trap. The former is a Shoot'Em Up platformer hybrid spinoff, while the latter is an Action RPG set immediately after Wonder Boy in Monster Land; with the prologue taking place in the Very Definitely Final Dungeon of that game.
 * Monster World IV has mostly the same gameplay as the previous Wonder Boy / Monster World titles, but a completely unrelated story.

Role-Playing Games

 * The Final Fantasy series, with a few exceptions centered around Final Fantasy VII and Final Fantasy X, making its use of numbers somewhat strange.
 * Square Enix's Ivalice realm is a consistent world visited by the player at various different point in history, each time centered on completely unrelated protagonists. The titles are non linear; the Final Fantasy Tactics games, Vagrant Story and Final Fantasy XII.
 * That said, even Ivalice's continuity is a little haphazard. Back when they were new, the only indication of a connection between Final Fantasy Tactics and Vagrant Story was a few small references; FFT's own sequel later on, Final Fantasy Tactics Advance, also had very little to do with its predecessor. Making Ivalice a more explicitly consistent setting was a later development, and it's still a bit different than it started out since it took some retconning to fit things together.
 * However, some games are now getting sequels, probably inspired by the success of the Compilation of Final Fantasy VII. For example, Final Fantasy IV has a cellphone/WiiWare game sequel, Final Fantasy IV the After Years.
 * And as of Dissidia Final Fantasy, the entire series takes place in a larger Multiverse. Provided Dissidia is Canon, anyway.
 * Final Fantasy IX already hinted at a multiverse; Dissidia just elaborated on it.
 * As did Final Fantasy X-2. The Ultimania notes that
 * A Japanese guide for Final Fantasy II (Final Fantasy IV Settei Shiryou Hen, for those curious) states that the world of that game is the same of the Blue Planet, only 100 years beforehand. It also states that the Deathbringer that King Fabul gives to Cecil was left to him by Leon from II, who then renounced his status as a Dark Knight to become a priest (as a parallel to Cecil's own class change from Dark Knight to Paladin). However, this is possibly shot down again in Final Fantasy IV the After Years, which reverts the whole "Kain being Ricard's son" thing back to a Retroactive Legacy Mythology Gag; there, Kain says that he was roughly the same age as Ceodore (17) when he heard news, while the Kain seen in II is a young boy.
 * Final Fantasy XIII is the first game to be made with other games (Final Fantasy Versus XIII for example) in the same setting in mind. However they have nothing to do with each other for the most part, with the exception with the direct sequel Final Fantasy XIII-2.
 * Likewise, there are only two continuities in the Tales (series), non-canonical crossovers Tales of the World notwithstanding. The first is the "Aselia" timeline, encompassing two Symphonia games and Phantasia. All of them have different protagonists (the Symphonia sequel has a different protagonist, and both are set around 4000 years before Phantasia). The second is the Destiny timeline, the second game being about the son of the heroes of the first. In all cases, no Bag of Spilling is invoked.
 * Due to an unusual twist of plot, Valkyrie Profile 2: Silmeria comes both before and after Valkyrie Profile.
 * And Covenant Of The Plume is a prequel. I suppose when the first game features the end of the world, there's nowhere to go but backward.
 * Breath of Fire has a remarkably similar situation going to Zelda, only complicated by two, possibly three, canonical Alternate Universe scenarios:
 * Breath of Fire I-III are canonically (per Word Of Capcom and storyline) in the same universe but separated by millenia between games.
 * Breath of Fire V, aka Dragon Quarter, is (per Word Of Capcom) in its own Alternate Universe completely unrelated to other games in the series. The gameplay is also radically different, to the point it actually resulted in a Broken Base within that fandom.
 * The real mess comes in sorting out where Breath of Fire IV fits, because there is no Word Of Capcom where the game fits, the game was rushed to production to outrun Author Existence Failure, and subsequent supplementary material (like the artbook, a Novelization, a second Breath of Fire Complete Works artbook, no less than two side-stories that were released for Japanese smartphones, and finally a manga that relied heavily on info from the artbook and Capcom's production staff) have done absolutely nothing to clarify this. As a result, a lot of Fanon and occasional Fan Wank occurs as to whether IV is an Alternate Universe or a Non-Linear Sequel (with the most popular Fanon Timeline going towards it being a Non-Linear Sequel that is a prequel to I-III).
 * The first two Lufia games where placed in reverse order, with Lufia II happening before Lufia I. The first game even started with A Taste of Power that eventually became the final conflict in the second game, creating a nice little loop of continuity.
 * Konami also has another series that deals with this particular trope: Suikoden. The games all take place in the same world (except for Tierkreis), but all take place in different regions and, more importantly, different times. To be specific, the first game to occur chronologically was Suikoden IV. One hundred and fifty years later, we experience the events of Suikoden V, then we get the original Suikoden I five years later, with Suikoden II following it up with the shortest time span between games (a mere three years), then we get Suikoden III 15 years after that.
 * One strange thing about this series is the character Viki, who is well-known for not only teleporting to different regions, but through time as well (though this is not intentional, as she just has very bad luck and is a huge klutz). What's interesting is that she teleports through the games in numerical order, rather than chronologically. In other words, she teleports from the events of Suikoden I to Suikoden II, then to Suikoden III, and so on.
 * Viki teleporting in numerical order is debatable. There are events that occur in Suikogaiden with Viki that, if she moved in numerical order, would imply her having knowledge of events that occurred (or knowing people she met) in earlier games. Her lack of knowledge of who Lorelai is in Suikoden V, even though they are both members of the Hero's army in I and II, is telling.
 * That, and there are two different versions of Viki in III.
 * The Grandia series developed by Game Arts is similar to Final Fantasy in that the games share little except for the battle system.
 * Fable II is set 500 years after Fable I, with a different protagonist.
 * Each MOTHER game is set in a different time period, with mostly new characters and locations. In fact, the only connection between the first two is the name of the Big Bad, Giegue/Gyiyg/Giygas, plus a few items and attack names. The connection between the second and third seems even more tenuous, going from a good-natured parody of modern America to a mysterious frontier town in the "Nowhere Islands",
 * The Wild Arms games were assumed to be non-linear at first, but Word of God says that they all take place on the same planet. This doesn't make much sense.
 * If that's the case, the games take place at least multiple millennia apart with Fridge Logic mass-reconfiguration of the planet's continental crust fragments. But hey, if the planet's center is filled with supernatural blue mud, you can Hand Wave just about anything.
 * Actually, it was set in the same "universe" but not the same planet.
 * The Elder Scrolls series all take place in the same world and are roughly chronological, but have different locations and, for the most part, characters.
 * Except for the spin-offs. Battlespire takes place just before and during the first game, while Redguard takes place roughly 300 years before the first game.
 * The up-coming Skyrim is going to take place two centuries after Oblivion, but has been announced as a more direct sequel, events-wise.
 * Dragon Quest: Several titles are taken place in the same universe with similar past events. Dragon Quest III is the prequel of the first two games, which are collectively known as the "Loto Trilogy" or "Erdrick Trilogy" (depending on your translation).
 * The fourth, fifth, and sixth games are also part of their own saga (the loosely-connected Zenithian trilogy), albeit in Anachronic Order.
 * To date, only Dragon Quest VII and Dragon Quest IX seem to be standalone titles, as Dragon Quest VIII hints at a multiverse due to Torneko Taloon from IV also cameos as an opponent in Morrie's Monster Arena in VIII. Just to complicate matters, IX features character cameos and cosplay gear from all eight previous games and the Dragon Quest Swords spinoff, which may mean something or may just be Mythology Gags. Then there's the early Dragon Quest Monsters games - one's a prequel to VI, another had a VII character wind up in the Erdrick Trilogy world long after the events of I - III.
 * The continuity between the World of Mana games is rather loose. At least one game, Legend of Mana, has been declared non-canon by Word of God, and the mobile phone game Friends of Mana takes place in a totally different world.
 * Pokémon does this. Not counting Gold and Silver, which were direct sequels to the first games, Red and Blue, every iteration since has been part of a jumbled up timeline that can only be explained by in-game functions.
 * It goes like this: Pokémon Red and Blue start off the series. At the same time that these are happening, the events of Pokémon Ruby and Sapphire, games released six years later as part of the third generation, take place. Three years later, the events of Pokémon Gold and Silver take place. A very short time afterwards, the events of Pokémon Diamond and Pearl occur. (It helps if you consider the remakes; Ruby and Sapphire and FireRed and LeafGreen are part of the same generation and take place at roughly the same time, as are Diamond and Pearl and HeartGold and SoulSilver.) And then, some unknown, as of right now, time later, (likely multiple years) the events of Pokémon Black and White take place. Pokémon Colosseum and Pokémon XD go wherever you want them to.
 * Until now, when Pokémon Black 2 and White 2 are coming out.
 * Dark Souls, despite being a Spiritual Sequel, is this to Demons Souls. The game replaces the Hub Level and level select style of Demons Souls with a world design heavily reminiscent of 3D Metroidvania games like Metroid Prime and Legacy of Kain: Soul Reaver.
 * Most of the Shin Megami Tensei sequels are like this, the major exceptions being Shin Megami Tensei II, the second Digital Devil Saga game, and the Raidou Kuzunoha games. Some games are hinted/confirmed to take place in the same continuity as their predecessors (Shin Megami Tensei III Nocturne, Persona4) but feature completely different plotlines and characters, with only a handful of Call Backs connecting them.

Shoot Em Ups

 * Contra 4 was titled as such to indicate its placement as a sequel to Contra III: The Alien Wars. It is actually the eleventh original installment in the series and the second one specifically made for a portable platform (following Operation C on the original Game Boy). The game got away with this due to the fact that none of the other Contra sequels between III and 4 were numbered (the actual game that followed Contra III was Contra: Hard Corps for the Sega Genesis).
 * The Aleste series kept a few main characters constant in its earlier installments, most of which could probably be linked together in order. Then there's Power Strike II for the Sega Master System and Robo Aleste for the Sega CD, each of which is set in a different historical time period with different characters.

Simulation Games

 * Wing Commander had a few of these. Wing Commander II was set ten years after the end of the second addon, the X Box Live game Wing Commander Armada was set 20 years after the events of Wing Commander Prophecy, and Privateer 2: The Darkening was... well, its own little world, for the most part, with subtle hints of a connection to the "main" games dropped throughout the game.

Stealth-Based Games

 * All the numbered Metal Gear sequels followed a linear chronology with one notable exception: Metal Gear Solid 3 Snake Eater serves as a prequel to the entire Metal Gear canon, being set decades before the events of the very first Metal Gear 1987. Metal Gear Solid Portable Ops and Metal Gear Solid Peace Walker, both for the PSP, serve as sequels to the plot of MGS3 and follow Big Boss' further adventures before the first Metal Gear.

Survival Horror

 * The Resident Evil games release order mostly matches the chronology of the series, except for the prequel Resident Evil 0 and, weirdly, Resident Evil 3 Nemesis, which takes place both before and after Resident Evil 2 (Resident Evil Code Veronica, despite not having a numbered title, is the true continuation to the events of Resident Evil 2).
 * The spin-offs are a bit looser with chronology -- the two Outbreak games take place roughly concurrently with 2 and 3, the final chapter of Umbrella Chronicles takes place a year before 4, and Darkside Chronicles is set to include a scenario showing Leon in South America two years before 4 that'll show what happened between Leon and.
 * The 3DS game, Resident Evil Revelations, is set before RE5 and Resident Evil: Operation Raccoon City takes place just before and during Resident Evil 2.

Third-Person Shooter

 * Dead to Rights was released in 2002 and follows Jack Slate, a cop in Grant City. Dead to Rights 2 was released in 2005, and despite it being a numbered actually takes place before the original game. A third game Dead to Rights: Reckoning was released for the PSP shortly after #2. It actually precedes 2. There is a fourth game announced for release in 2010. With luck, it will feature Jack as a fetus.
 * Judging from the Wikipedia page (and change this if I'm wrong) it seems to be a remake with a slightly different storyline of the first game. So, Yeah, four "sequels" and we haven't even got past the first storyline. Whoopie...
 * The Armored Core series is a great example of this. Each successive game takes place several years, sometimes even decades after previous one and casts the player in the role of a new AFGNCAAP with no connection to the previous one. Usually the presence of a number in the title indicates a Continuity Reboot. So far, Armored Core, Project Phantasma, Master of Arena, Armored Core 2 and Another Age take place in one continuity. Armored Core 3, Silent Line, Nexus,  Nine Breaker , and Last Raven take place in a second continuity. Armored Core 4 and for Answer take place in a third. And it's yet to be seen where Armored Core 5 will fit in.
 * There's also the Gaiden Game Formula Front, which uses elements from the Armored Core 3 universe, but it clearly not set in it. It's really just a Fighting Game with Robots... No, not that one.

Turn-Based Strategy

 * The Fire Emblem series has, as of the eleventh installment, five different Verses, each with their own characters, settings, mythologies, and plots. The MacGuffin in each universe is almost always the titular Fire Emblem, although it's called something completely different except for one line thrown in about how some people (who you'll never meet) call it the Fire Emblem.
 * Word of God claims the Jugdral games take place in the same universe as the Akaneia games, but 1000 years prior. Other than that, there are no links between 'Verses. Sacred Stones remains the only game in the series not linked to any other, though it is a Spiritual Successor to Gaiden.
 * Disgaea, though there are a few recurring characters.

Wide Open Sandbox

 * The PS2-era Grand Theft Auto game chronology goes: Vice City Stories (set in 1984; released 2006), Vice City (1986; 2002), San Andreas (1992; 2004), Liberty City Stories (1998; 2005), Grand Theft Auto Advance (2000; 2004), Grand Theft Auto III (2001; 2001).
 * From Grand Theft Auto 4 onwards, the games are effectively set in Another Dimension.

Non-video Game Examples

 * Chaos Fighters II has nothing to do with Chaos Fighters except with the magic and weaponry system.
 * The Chronicles of Narnia was written with each story assumed to be the last (as admitted by the author) so as such several of the later ones answer questions rather then continuing the story. As such The Magician's Nephew was written and published 6th but takes place first. The Lion, the Witch, and the Wardrobe which was first is second. A horse and His Boy was written fourth, published fifth, and set during a timeskip in LW&W. Though with those three sorted out the rest follow order of publication: Prince Caspian (Which is set centuries after LW&W), Voyage of the Dawn Treader, Silver Chair, and Last Battle. Plus in world the time shifts because Narnia is a parallel world not perfectly synced with ours.
 * The Scorpion King is a prequel to The Mummy Trilogy, the second film of which introduced the scorpion king and showed his final fate. They then made The Scorpion King 2, which was a prequel to the prequel. There are also plans for The Scorpion King 3 which will be a sequel to the prequel of the prequel.
 * The "Road Pictures" of Bob Hope and Bing Crosby. Hope, Crosby and Dorothy Lamour play different characters with different names in each, and at least one is set in a wildly different time period, but they all have the same cast, the same style of madcap humor, and much the same formula. It is common for there to be references to their past adventures, albeit usually during one of the many instances of Breaking the Fourth Wall.
 * Kamen Rider Agito explicitly takes place in the same universe as Kamen Rider Kuuga; the only on-screen references to this are a couple of off-hand mentions of the Grongi (Kuuga's enemies) and the G3 suit being built by the police based off of data taken on "Unidentified Lifeform #4" (the Tokyo Police's callname for Kuuga). There was also said to be a Great Ofscreen War where the Grongi were killed off by the Lords, Agito's enemies.
 * This all gets referenced in Kamen Rider Decade, where the Alternate Universe Agito World they visit is also an alternate Kuuga World, with the Great Offscreen War actively happening and an alternate version of Decade!Kuuga's Cool Big Sis mentor showing up as the head scientist behind the G3 Project.
 * Kamen Rider does this a lot, but more recently it's intensified. Kamen Rider Double makes an off-hand reference to Kamen Rider OOO's Transformation Trinket in one of the later episodes, and two of Kamen Rider Fourze's friends make a blink-and-you'll-miss-it cameo in the final episode of OOO. And on top of that, the Early-Bird Cameo pulled by OOO in Double's Big Damn Movie is explicitly referenced in the next movie, suggesting it's canon to both (as are, presumably, Kamen Rider Decade's encounter with Double and Fourze dropping in to help OOO with his movie's Big Bad).