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-> ''Some series formats (''Series/DoctorWho'', ''Series/QuantumLeap'') lend themselves particularly well to crossovers, since the primary texts involve a constant dislocation of the protagonists. The TARDIS has materialized everywhere from the Franchise/PlanetOfTheApes to Series/FawltyTowers, even on the set of ''Series/WheelOfFortune''. Sam Beckett finds himself occupying the bodies of everything from Martian invaders (''Franchise/TheWarOfTheWorlds'') to Franchise/IndianaJones and helping to resolve problems fans locate within their universes.''
-->-- '''Henry Jenkins''', ''Textual Poachers: Television Fans and Participatory Culture''

Sometimes a fanfic writer has to get really creative if they want to pull off a [[{{Crossover}} crossover fanfic]]. Sure, it's easy to do a crossover when the characters could feasibly come from the same world ([[LikeRealityUnlessNoted often modern Earth]]), but what about when they simply couldn't, such as each existing in a distinct ConstructedWorld? After all, you need to come up with ''some'' sort of justification for characters from two (or more) completely different universes meeting each other. Thankfully, there exist many pieces of media where the job of explaining how a crossover would work was already done for them by the original creators.

A specific species of FandomSpecificPlot, an Obvious Crossover Method is when the canon of a work supplies a concept that easily allows for potential crossover situations, which subsequently gets regular use in fan works to set up said crossovers. This is especially common with the ScienceFiction[=/=]ScienceFantasy genres, thanks to the abundance of {{Alternate Universe}}s, wormholes, {{Negative Space Wedgie}}s, and big galactic settings where the other works could be said to be hidden away in some other corner of the universe. A MutualMasquerade is another useful plot device, as are works with an IntrepidFictioneer premise.

Before adding an example, think if the method has been used frequently enough to qualify as a FandomSpecificPlot; if it doesn't, it doesn't go here.

Compare SailorEarth (a SisterTrope of a gap in the setting that can be filled with an OriginalCharacter, a minor character, or, overlapping with this, a crossover character), TheStationsOfTheCanon (which is more about a frequent side effect; this is about a possible cause).

----
!!Examples:

[[foldercontrol]]

[[folder:Anime & Manga]]
* ''Manga/AhMyGoddess'' crossovers can begin by simply having a character from a different franchise accidentally call the Goddess Technical Helpline (or the Earth Assistance Hotline or even the Demon Helpline) and being given a wish by one of the holy beings.
* ''Franchise/{{Digimon}}'': It is established in ''Anime/DigimonAdventure02'' that multiple universes exist, as well as multiple parallel Digital Worlds of the other various Digimon animes, with the various characters managing to [[IntercontinuityCrossover crossover with each other]] due to the Digital Worlds intersecting. As such, an easy way for most crossovers to happen is to have the universe's computers accidentally tap into (one of) the Digital Worlds.
* Since the wider universe in ''Anime/DragonBallZ'' is only vaguely hinted at, it's a simple matter to have Goku and/or Vegeta land on a crossover planet while they're out traveling in space.
* Using the Springtime Summoning Ritual in ''Literature/TheFamiliarOfZero'' to summon a crossover character is the basis of the majority of the series's fanfics in both English and Japanese. There is [[http://www35.atwiki.jp/anozero/ a Japanese wiki]] dedicated to compiling the fanfics from 2ch's thread on replacing Saito with other characters.
* The Gate from ''Anime/FullmetalAlchemist2003''. We know it leads to one parallel universe, so why not another?
* Instead of the eponymous ''Literature/{{Gate}}'' opening up in modern-day Tokyo and the Empire attempting to invade that it can literally open up anywhere else.
* ''Anime/HellGirl'' crosses over effortlessly with any halfway realistic series: just have a character get desperate and call on the rumoured Enma Ai for assistance. This probably explains its [[https://www.fanfiction.net/crossovers/***-Girl/3079/ massive crossover list]] on Website/FanFictionDotNet.
* Because the characters of ''Webcomic/HetaliaAxisPowers'' are supposed to exist in the real world but living in secret, it's quite easy to write a character meeting them. Bonus points for if that character's nation of origin/wherever they live at the moment is known, and/or if they were involved in a major historic event that a Nation would likely have been involved in.
* Since the characters in ''Franchise/LyricalNanoha'' can [[CasualInterplanetaryTravel transverse dimensions on a whim]], it's easy for fanfics to have Nanoha and Fate go on a mission in another franchise's universe.
* ''Anime/NeonGenesisEvangelion'':
** Fanfiction often has Shinji Ikari being raised by crossover characters (instead of his unnamed caretaker[[note]]teacher in the anime, uncle in the manga[[/note]]) before being called back by Gendo.
** Being pulled through the Dirac Sea to another (crossover) universe during the fight with Leliel.
* Zoro of ''Manga/OnePiece'' is a frequent source of MemeticMutation in regards to his infamous NoSenseOfDirection, namely writers keep putting him in many other franchises (not just anime, but films, books, even in ''real life'').
* ''Manga/RanmaOneHalf'':
** The Nanban Mirror from a filler episode allows TimeTravel crossovers.
** Before arriving at the Tendōs' home, Genma and Ranma visited or trained with some characters from the crossover series during their travels. Ranma being engaged to a character from the other series is not required, but frequently goes along with this.
** {{Revision}}ing Ranma's mother, Nodoka[[note]]or another secondary character with a sufficiently vague past[[/note]] as related to characters from the crossover series, which gets revealed in time for the plot to kick in.
** Also common: Ryōga [[NoSenseOfDirection wanders]] into another series.
** Rarer: Ranma, or another character (canonically Jusenkyō-cursed or otherwise), falls into the Spring of Drowned [Crossover Species] or [Crossover Character who ends up possessing the newly afflicted].
* ''Anime/SuperDimensionFortressMacross'' (and/or ''Anime/{{Robotech}}'') crossovers can have the SDF-1's first hyperspace fold take it even further than Pluto. (This can also be used to explain the final fates of Hikaru Ichijo and/or Rick Hunter in post-series fanfic.)
* It's easy to write a crossover with ''Manga/UruseiYatsura'' where Ataru is replaced as Lum's opponent in the tag game (and [[AccidentalProposal accidental fiancé]]) with a teenaged character from a different fandom.
* Pretty much every work has at least one crossover fic with ''Manga/XxxHolic'' where a character will come to Yuuko for a wish. The character's ability to come to her shop is usually explained by [[HandWave "Hitsuzen."]]
* The Space-Time Oscillation Bomb from ''Anime/SuperDimensionCenturyOrguss'', which fuses about a hundred parallel worlds into one singular world while constantly shifting worlds in and out.
** This was even officially done with Orguss's debut into the VideoGame/SuperRobotWars games, ''VideoGame/SuperRobotWarsZ'', in which the Space-Time Bomb merges the worlds of all the series in the game's roster together.
* If you have a character who is a martial artist, you can easily set a crossover with ''Manga/KenganAshura'' by have them participate in a Kengan Associacion match or tournament. In fact, this is how creators themselves set up crossover one-shots with ''Franchise/StreetFighter'' and ''Manga/BakiTheGrappler''. Bonus points for have any corporation from other side of the crossover be or trying to become members of the Associacion and/or any martial arts fighting ring having a rivarly or a working relationship with it.
[[/folder]]

[[folder:Comic Books]]
* {{The Multiverse}}s of [[Franchise/TheDCU DC]] and Franchise/{{Marvel|Universe}}, along with lesser-known examples like Creator/{{Wildstorm}}'s "Snowflake," are easy to stretch to include other crossover settings. (It's [[ComicBook/MarvelVersusDC kind of]] [[ComicBook/JLAAvengers canonical]] that the Marvel and DC multiverses include each other.)
* [[Franchise/GreenLantern Power rings]]. They can fly to any character from space and recruit them, tends to [[ClingyMacGuffin cling onto the chosen whether they want it or not]], and is attracted to powerful quantities of common qualities among fictional characters. For example:
--> "[[WesternAnimation/AvatarTheLastAirbender Zuko of the Fire Nation]]. You have [[BurningWithAnger great rage]] in your heart. You belong to the Red Lantern Corps."\\
"[[WesternAnimation/TheSimpsons Charles Burns of Springfield]]. You [[CorruptCorporateExecutive want it all]]."\\
"[[VideoGame/RuneScape Zaros of Freneskae]]. You have the ability to [[TheDreaded instill great fear]]. Welcome to the Sinestro Corps."\\
"UsefulNotes/TheodoreRoosevelt of Earth. You have the ability to [[{{Determinator}} overcome great fear]]. Welcome to the Green Lantern Corps."
--> "[[Franchise/StarWars Luke Skywalker of Tatooine]]. You have the ability to [[HopeBringer instill great hope]]. Welcome to the Blue Lantern Corps."
--> "[[VideoGame/LeagueOfLegends Braum of Runeterra]]. You have the ability to [[AllLovingHero feel great compassion]]. Welcome to the Indigo Tribe."
--> "[[Franchise/{{Frozen}} Anna of Arendelle]]. You have [[ActOfTrueLove great love]] in your heart. Welcome to the Star Sapphires."
* ''ComicBook/{{Mickey Mouse|ComicUniverse}}'': In "A Mickey Mystery", Doctor Einmug displays an invention that lets one see what life is like on alternate Earths and later Mickey transfers to one such world. It has been argued that Doctor Einmug should be used if there is a point and not just as a vehicle for gadget, and so far the invention has only been used for a few "Disney-like" worlds. Imagine if the invention was used for other kinds of worlds as well.
* The Endless concept from ''ComicBook/TheSandman1989'' qualifies; given the nature of the cosmology in that plot, literally every universe ever imagined has existed, and the Endless represent fundamental concepts such as Death, Desire, Dreams, and Destiny. Usually the crossover is a brief encounter between a character and either Death or Dream, in that order of probability.
* ''ComicBook/{{Wolverine}}'':
** The biography of Wolverine easily allows [[CrossoverRelatives characters from other fandoms to be his descendants]], often ones he is unaware of. The comics already canonically did this with the introduction of [[Characters/WolverineSupportingCharacters Daken]], so this is just fans taking the idea to the next level.
** Another popular way for crossovers with Wolverine to happen is due to how long he's lived and will most likely continue to live longer. It's not uncommon for him to have met people either in the past or even the ''future''.
[[/folder]]

[[folder:Fan Works]]
* ''Fanfic/TheInfiniteLoops'': Half the point of the setting is to provide easy crossovers. [[GroundhogDayLoop Time is looping]], and sometimes loops involve crossing over to other universes. "Fused loops" can be everything from a handful of elements from one series showing up in another universe, to visiting loopers (with or without the natives Awake), to both universes being seamlessly merged for the duration.
[[/folder]]

[[folder:Films]]
* The basic concept of how [[TheMultiverse dimension-hopping]] works in ''Film/EverythingEverywhereAllAtOnce'' is an easy avenue for crossover fics where a character gains the experiences and skills of a character from another work played by the same actor.
* Given the nature of the films, it's easy to layer the plot of the ''Franchise/FinalDestination'' series onto nearly any other fandom; simply have one character experience a vision of them all dying in a terrible accident, have them save the others, and then have them be slowly hunted down by Death itself.
* Thanks to Freddy Krueger's [[DemonOfHumanOrigin nature]], [[DreamWeaver powers]] and already having [[Film/FreddyVsJason a few]] [[ComicBook/FreddyVsJasonVsAsh official]] [[VideoGame/MortalKombat9 crossovers]], it's not uncommon for the iconic killer of the ''Franchise/ANightmareOnElmStreet'' franchise to use his abilities to pop up in other franchises and menace certain characters in their dreams.
* ''Franchise/StarWars'' takes place in a large, old galaxy with lots of obscure backwaters, lost history, and even unexplored space. As such, it's easy to take a series that's limited to one or just a few planets and drop it into the ''Star Wars'' galaxy, allowing ''Star Wars'' characters to land there or abduct others and take them back to the wider galaxy. Heck, the official Databank on starwars.com did it at one point as an [[AprilFoolsDay April Fools joke]], adding entries for characters from ''Film/{{Willow}}'' written as if they lived on the planet Andowyne in the ''Star Wars'' galaxy.
* ''Film/StayTuned'' has great crossover potential, as Hellvision has [[NumberOfTheBeast 666]] channels, but only a few of them appear in the film. One could initiate a crossover by simply having the Knables (or only Roy or Helen) slip into yet another channel that just happens to be broadcasting a rerun of another movie or a TV show.
[[/folder]]

[[folder:Literature]]
* ''Literature/{{Animorphs}}'':
** Elfangor lands somewhere else on Earth and gives morphing to a different group of characters (if it's not on Earth, then the Yeerks colonized a different planet that has humans).
** Messing around with the [[TimeyWimeyBall Time Matrix]] transports the Animorphs to another universe.
* ''Literature/{{Discworld}}'':
** Dimensional instability caused by magical accidents can lead to unexpected characters popping up from any other reality.
** Any setting with a library is connected to L-Space, and therefore to the Library of Unseen University.
** A popular concept is to have a canonically dead character from another work get picked up by Discworld's Death.
* ''Franchise/HarryPotter'':
** A crossover character raises Harry instead of his aunt and uncle.
*** A crossover character turns out to be a distant (or not) relative of Harry's that the Dursleys never talk about.
** Falling through the Veil from ''Literature/HarryPotterAndTheOrderOfThePhoenix'', allowing other characters to enter the Potter-verse and Potter-verse characters to land in other worlds.
** For a fantasy series, having the central characters either attend or teach at Hogwarts (particularly on Defense Against the Dark Arts as each book always needs a new teacher for this subject). This includes series set in worlds that are [[ConstructedWorld fundamentally]] [[FictionalEarth incompatible]] with [[LikeRealityUnlessNoted Harry Potter's]].
** For a series with teen protagonists, they attend Hogwarts as [[NewTransferStudent "transfer students,"]] and are conveniently placed in Harry's year.
** Since one of the major premises of the ''Harry Potter'' franchise is that the wizarding world is hidden within the "real world," crossover characters could be [[BrokenMasquerade exposed]] to the {{Masquerade}}.
*** A specific subtype for series that have their own {{masquerade}} will have the characters try to infiltrate the wizarding world while maintaining their own secrecy until inevitably they are exposed as well, usually either by Dumbledore or someone in Harry's circle of friends.
* ''Literature/{{Worm}}'':
** ''Worm'' features a diverse range of superpowers, the origins of which are not revealed until late in the story. Therefore, a common idea is that superpowers are caused by [[TabletopGame/{{Exalted}} The Incarnae]], [[Series/BabylonFive Vorlons]], or what have you.
** Speaking of that diverse range of superpowers, one individual can create doorways between alternate universes. This multiverse opens up a wide variety of crossover possibilities—especially since WordOfGod suggests that ''Literature/{{Pact}}'', ''Literature/{{Twig}}'', and the various snippets written by Wildbow exist elsewhere in the ''Worm'' multiverse.
** One FandomSpecificPlot that occurs frequently in the ''Worm'' fandom is the Altpower fic, in which the main character is given a different superpower than in canon and the consequences of that are explored. This is frequently exploited to bring in powers and plot elements native to another work.
** Taylor, after getting shot by Contessa, is reincarnated in/moved to another world.
[[/folder]]

[[folder:Live-Action TV]]
* ''Series/BattlestarGalactica2003'' crossover fics often involve the Earth that the Colonial Fleet is looking for turning out to be Earth as it is in the other setting.
* ''Series/TheBigBangTheory'': Several crossover stories involve characters from other franchises getting jobs at Caltech, often working as lab assistants to the protagonists.
* The witches of ''Series/{{Bewitched}}'' can manifest fictional characters from any continuity and transport themselves into fictional realms.
* The ''Series/BlackMirror'' episode "San Junipero" features [[spoiler:a virtual reality that stores the consciousness of dead people and where the living can visit for a few hours]]. Any story you want about revisiting a character that's dead, there you go.
* ''Series/BuffyTheVampireSlayer'' has at least five:
** "YAHF" (Yet Another Halloween Fic) -- the many, many stories based on [[Recap/BuffyTheVampireSlayerS2E6Halloween the second season Halloween episode]] where a spell makes everyone [[BecomingTheCostume Become the Costume]].[[note]]For instance, Buffy turns into an 18th-century noblewoman because she picked a PimpedOutDress, while Willow's BedsheetGhost outfit leaves her walking through walls.[[/note]] Naturally, a crossover is as simple as changing which costumes TheLittleShopThatWasntThereYesterday had in stock that day. It is even simpler when the crossover character is a ghost, noblewoman or soldier, since that is what Willow, Buffy and Xander respectively went as that Halloween. Xander especially is [[FandomSpecificPlot a popular choice]] to turn into specific soldiers from other works and franchises, given both the generic nature of his costume and the fact that, unlike the other characters, his costume left behind BrainwashResidue that can be used to continue the story well after Halloween.
** Buffy jumping through a portal at the end of the fifth season causes her to wind up in the world of another story, instead of dying as in canon.
** For a variation on the above, a character from another story will come out of the portal and take Buffy's place.
** The "Slayer Speculation Fic," where a crossover female character gets called as the Slayer.
** Fics where Sweet the musical demon from [[Recap/BuffyTheVampireSlayerS6E7OnceMoreWithFeeling "Once More, with Feeling"]] comes to the setting of another show and makes that show's characters act out their feelings in song and dance like he did to the ''Buffy'' cast when he was summoned from the underworld to Sunnydale. All that's needed is for someone in the setting to find his amulet, or even for him to just feel like dropping in on another dimension.
* Creator/JensenAckles played two characters on ''Series/DarkAngel'' a few years before playing Dean Winchester on ''Series/{{Supernatural}}'', SerialKiller Ben and LovableRogue Alec, who were genetically engineered twin [[ClonesArePeopleToo clones]], which naturally lends itself to crossovers where Dean was the source of their DNA.
* ''Series/DarkMatter2015'' crossovers can be justified by a malfunction with the ''Raza'''s Blink Drive taking the crew to another universe (something that happened once on the show itself). In fact, a ''Dark Matter''/''[[Franchise/StargateVerse Stargate]]'' crossover using this conceit came ''very'' close to materializing in canon when MGM's Stargate Command streaming service was launched (both franchises had Creator/JosephMallozzi and his longtime writing partner Creator/PaulMullie as showrunners).
* Since ''Series/DoctorWho'' can theoretically take place anywhere in time and space, it's pretty easy for it to have crossovers with anything just by having the TARDIS land there. Even in cases where the other work's universe conflicts with that of ''Doctor Who'', there have been rare occasions in canon in which the TARDIS has traveled to an AlternateUniverse due to a glitch, or else the TimeyWimeyBall can be used to explain that history has been rewritten to match the other series. In any case, the ''Doctor Who'' canon is itself incredibly fluid (read: [[ContinuitySnarl tangled]] and contradictory) to begin with, meaning that potential readers familiar to the series are used to papering over cracks.
** Which is to say nothing of the possibilities opened up by involving the Doctor's sometime associate and parody Franchise/IrisWildthyme, who originated outside ''Doctor Who'' continuity before being transplanted into the Doctor's [[Franchise/DoctorWhoExpandedUniverse Expanded Universe]] and crosses continuities with ease in her smaller-on-the-inside Celestial Omnibus.
* Season 2 of ''Series/TheFlash2014'' features the black hole caused by the Reverse-Flash's erasure from the timeline opening portals to an alternate dimension, and it's explicitly stated that there are a potentially infinite number of other alternate worlds that could be accessed. In fact, in less than a year the show started using this method to ''canonically'' [[CanonWelding connect the 'verse to previously unrelated stories]] like ''Series/{{Supergirl}}''.
* ''Series/{{Farscape}}''. Once Crichton figures out wormholes, and wormholes are revealed to possibly lead to alternate universes, pretty much anything is fair game.
* ''Series/{{Fringe}}'' has alternate world travel in the later seasons.
* ''Series/{{Highlander}}'' is like this because of the whole concept is that immortals exist in secret among regular humans. Any series that's reasonably reality-based (and even a few that aren't) can be crossed with it, including any set in the distant past or far future. Any pre-existing character can also be a secret immortal (as long as they haven't had biological kids, due to the ImmortalProcreationClause). Anyone adopted and/or badly injured onscreen (the "violent first death" mechanic for immortals) is especially attractive since immortals are all foundlings and retcons of adoption aren’t hard for those with canonical parents.
* ''Series/KamenRiderDecade'' has the plot of "[[TheHero Tsukasa]] and his friends travel from [[AlternateUniverse world]] [[AdventureTowns to world]]," so needless to say there's plenty of scope. Furthermore, if you take Decade's crossover movie with W and ''Super Hero Taisen'' as canon, pretty much every Japanese live-action superhero owned by Toei can cross over with each other. It doesn't help that after their trip to the Nine Rider Worlds, they got to a whole new set of worlds, one of which [[Series/SamuraiSentaiShinkenger lacked any Kamen Riders at all]]. And considering how their travels are random ''and'' there's an AndTheAdventureContinues moment in the GrandFinale...
* Given the nature of ''Series/MurderSheWrote'', it's easy to write a crossover between it and any other series by having Jessica Fletcher solve a murder mystery from another series or film while on one of her trips, or by having a murderer from another series or film wind up in Cabot Cove.
* ''Series/OnceUponATime'' establishes that there are a plethora of realms out there, including Wonderland, Neverland, Oz, and the realm most of the characters originally came from, which is an expansive world that consists of various settings ranging from [[WesternAnimation/{{Aladdin}} Agrabah]] to [[Franchise/{{Frozen}} Arendelle]] to Camelot to [[WesternAnimation/{{Brave}} DunBroch]] to the Enchanted Forest from various fairy tales. And our world is also among the list of realms. Couple that with various methods of travel established in the show and you got yourself an instant crossover.
* ''Series/QuantumLeap'' runs on HandWave that doesn't necessarily strictly limit itself to one and the same universe. That way, Sam can leap into just about ''anyone'' from ''any'' franchise. Lots of ''Quantum Leap'' fanfics actually cross other franchises over with ''Quantum Leap'' (in that order).
* The sliding device from ''Series/{{Sliders}}'' simply drops the characters in the crossover world.
* ''Franchise/{{Stargate|Verse}}'':
** Malfunctioning Stargates, or simply a new dialing connection, lands the ''Stargate'' characters in another franchise. Alternately, technology or magic from the other side opens a portal that spits out the characters in Stargate Command. Alternate world travel and time travel are also in-canon options.
** Given that humans seem to be spread across both the Milky Way and [[Series/StargateAtlantis Pegasus]] galaxies, it's also possible for fanfics to make the characters of other franchises inhabit different planets within either of the two galaxies and have them meet either SG-1 or the Atlantis team via Stargate.
* ''Franchise/StarTrek'' is so full of {{Negative Space Wedgie}}s, [[SwirlyEnergyThingy Swirly Energy Thingies]], {{Teleporter Accident}}s, {{Holodeck Malfunction}}s, and other {{technobabble}} that it's practically a grab bag for crossovers. Plus the SufficientlyAdvancedAliens. For example, the ''Franchise/StarWars'' crossover ''Fanfic/{{Conquest}}'' used a wormhole, with Q constantly implying he had a hand in that.
* ''Series/{{Supernatural}}'' can have [[AWizardDidIt an angel or a supernatural object]] transport characters into other works. Examples of both, leading to TrappedInTVLand scenarios such as the ''Franchise/ScoobyDoo'' crossover episode [[Recap/SupernaturalS13E16ScoobyNatural "[=ScoobyNatural=]"]], are canon. Alternate universes are also canon (as are ways to access them), including one where ''Supernatural'' is a TV show, which makes crossing over with a setting that has [[ReferencedBy/{{Supernatural}} referenced SPN as fictional]] less of an issue.
[[/folder]]

[[folder:Tabletop Games]]
* ''TabletopGame/DungeonsAndDragons'':
** ''TabletopGame/{{Spelljammer}}'' was already doing this to various worlds within the franchise by establishing they all exist within Crystal Spheres, which can as easily contain area the size of single star system or whole galaxies, each with its own laws of physics, connected by traversable [[SpaceIsAnOcean sea of stars]] (phlogiston in 2e, Astral Plane in 5e). Spelljammer fandom ran with the idea and you will see them discussing physics of "Setting-space" for any world of any game or work of fiction, and how to integrate them into wider universe.
** ''TabletopGame/{{Planescape}}'' pretty much does the same, thanks to Sigil - the city of infinite portals leading to all kinds of worlds and planes. The place itself is booming with all kinds of creatures from every corner of the multiverse - beings every world on material plane, demons, devils, celestials, genies, fey and aberrations coming it, doing trade, travelling somewhere else and so on.
** ''TabletopGame/{{Ravenloft}}'' has Demiplane of Dread, which is collection of Domains, each ruled by and serving as a prison to a Darklord, an individual who comitted acts so evil the reality itself rejected them and Dark Powers claimed them to psychologically torture for eternity. You could theoretically have any particurally vile antagonist from any series, even those who should be dead, wake up in their own tailor-made hell, where they have absolute power at the cost of psychological torture and pesky adventurers showing up to kill them. Because it is possible to have people sucked into Demiplane of Dread as easily as by walking into strange mist, meaning any hero could show up to fight existing Darklord as well.
** Feywild, realm of TheFairFolk is said to connect even the worlds isolated from other means. Even official books recommend using Feywild to explain how a character could play a Kender outside of [[TabletopGame/{{Dragonlance}} Krynn]] or characters from, normally cut-off from rest of the Multiverse, world of ''TabletopGame/{{Mystara}}'' make a cameo in ''TabletopGame/TheWildBeyondTheWitchlight''. As such any setting with the Fey in it can potentially be connected to Feywild as well, allowing characters to walk in and out.
** ''Journeys Through the Radiant Citadel'' introduces the titular Citadel, which connects fifteen cultures on distant worlds. But it is said that in the past it connected total of ''twenty seven'' of those and people in charge are trying to rediscover the missing twelve, among which could be any world of the game master or writer's choice.
* The ''TabletopGame/{{GURPS}}'' setting ''TabletopGame/InfiniteWorlds'' was created with crossovers in mind, as pretty much every setting it's possible to emulate with a ''GURPS'' sourcebook has its own parallel world listed (including the licensed ones).
* ''TabletopGame/MagicTheGathering'' is specifically about people who can travel from one world to another; claiming that ''any'' given fictional setting is within the Planes of the Multiverse is not hard, and since ''anyone'' could potentially have the Planeswalker Spark without knowing it, it's easy enough to have a given character ignite.
* The world of ''TabletopGame/{{Rifts}}'' was practically ''built'' for crossovers -- one never knows where a Rift might lead.
* ''TabletopGame/{{TORG}}'' was also built for crossovers, since the main premise of the game is that {{alternate universe}}s are "invading our world" and overwriting it.
* ''TabletopGame/{{Warhammer 40000}}'' crossovers usually involve a mishap with the [[HyperspaceIsAScaryPlace warp]]. Also, there are stated to be gaps between the systems of the Imperium where an uncontacted planet, or even an entire small to medium-sized (human or not) civilisation, can fit in. And since Primarchs' pods happened to land on a planet, it is a reasonably popular idea to claim that a great hero of such a crossed-over franchise was one of the Lost Primarchs; a popular assumption in the fandom is that Sigmar of the sister franchise Warhammer Fantasy was exactly that.
[[/folder]]

[[folder:Video Games]]
* As Chris Sims of Comics Alliance remarked, "If Robin Hood served under King Richard I during the Third Crusade, then he and Altair from ''Franchise/AssassinsCreed'' were in the same place at the same time. [[FanficFuel You can have that one for free, fan-fiction writers.]]"
* ''VideoGame/BioShockInfinite'', and by extension [[VideoGame/BioShock the rest of the series]], thanks to the infinite worlds shown at the end of the game.
* Due to their nature of being a FeaturelessProtagonist, it's easy to replace ''VideoGame/BlueArchive'''s Sensei with other characters.
* ''VideoGame/DissidiaFinalFantasy'': Pick a number from 1 to 11. This is the cycle in which your story takes place, as only the twelfth and thirteenth cycles are shown in canon. Next, add whichever characters you like. Finally, write your story, making sure to kill off or otherwise write out the new characters at the end, so that they don't appear in canon. Alternatively, as per ''[[VideoGame/DissidiaFinalFantasy2015 NT]]'', you can just slot any new faces into [[SoftReboot that game's]] conflict, as it's a DistantSequel to the original conflict between Cosmos and Chaos (only with their successors taking up the mantle) wherein the ''Franchise/FinalFantasy'' characters are usually pulled from a point both after their canonical journeys and the thirteenth cycle.
* ''VideoGame/Doom2016'' ended with Samuel Hayden betraying the Doom Slayer and VEGA and teleporting them away from Earth, only for them to return a few years later in ''VideoGame/DoomEternal''. Several crossovers start with the pair being teleported away and then landing on another planet or universe.
* ''VideoGame/TheElderScrollsVSkyrim'': The "[[https://knowyourmeme.com/memes/toddroll-youre-finally-awake Toddroll]]" meme ("Hey, you, you're finally awake.") started by shitposting ''VideoGame/Fallout76'' as a StealthSequel, and paved the way to popularize the idea of characters from any franchise [[YouAllMeetInACell waking up in the cart]] from the start of ''Skyrim'' after being knocked unconscious for ''any'' reason.
* ''VideoGame/FalloutNewVegas'':
** The Big Mountain Transportalponder from [[DownloadableContent Old World Blues]] malfunctioning because the Think Tank from Big MT are incompetent seems to be a popular option to get the {{Courier}} into whatever universe you want him/her to be.
*** ''[[FanFic/MassFoundations Redemption in the Stars]]'' has Ethan Sunderland, this fic's interpretation of the Courier, fall onto this trope, thus making Ethan's motivation to find a way back home, though it was a self-imposed error in improving the Transportalponder and Big Mountain being Big Mountain.
* ''VideoGame/FireEmblemAwakening'' has the Outrealm Gate, which can transport the characters to other worlds and alternate timelines. This is to the extent of being an [[InUniverse in-universe example]], as one of the main purposes of the Outrealm Gate is that it serves to justify the DLC maps, many of which are crossovers with previous games in [[Franchise/FireEmblem the series]]. ''VideoGame/FireEmblemFates'' would expand upon [[TheMultiverse this concept]] (particularly since [[spoiler:three of the Nohrian characters are returning second generation units from ''Awakening'' under new aliases, having crossed over at the behest of the PlayerCharacter's father]]), and later games and media (''[[VideoGame/FireEmblemHeroes Heroes]]'', ''[[TabletopGame/FireEmblemCipher Cipher]]'', ''[[VideoGame/FireEmblemWarriors Warriors]]'', etc.) implicitly make use of Outrealm Gates to justify previously established characters popping up in an OriginalGeneration setting (or, in the case of two DLC maps for ''[[VideoGame/FireEmblemGaiden Shadows of Valentia]]'', [[CanonImmigrant the exact opposite]]). This is even canonical within the greater Creator/{{Nintendo}} library of [=IPs=], as [[VideoGame/FireEmblemAwakening Robin, Lucina]], and [[VideoGame/FireEmblemFates Corrin's]] appearances in the [[VideoGame/SuperSmashBrosForNintendo3DSAndWiiU fourth]] ''VideoGame/SuperSmashBros'' are officially {{Alternate Sel|f}}ves of those characters (with Corrin's reveal trailer having him [[TakeAThirdOption take a fourth option]] during the Branch of Fate that would normally lead to ''Birthright'', ''Conquest'', or ''Revelation'' and decide to join ''Smash'' instead).
* ''VideoGame/FiveNightsAtFreddys'' has at least two of these:
** First, there's the use of other SuckECheeses (or hell, ''any'' restaurant) from other forms of media. Fans have taken these locations and made "fan games" out of them, essentially remaking the original game using characters from the latter media. For example: ''Five Nights at the [[WesternAnimation/SpongeBobSquarePants Krusty Krab]]''.
** A somewhat less common--but still used--idea is taking pretty much ''any'' character from ''any'' medium and making them a security guard at Freddy's. The idea is to see how ''they'' would handle the situation, whether it be cowering in the corner like a crybaby, or giving those animatronics a piece of their mind (usually by taking away pieces of ''the robots''). ''FanFic/DantesNightAtFreddys'' is one such example. In a more lighthearted and friendly note, some can find out what is going on with said animatronics[[labelnote:*]]Unsurprisingly the knowledge of kids being killed and possessing animatronics can illicit some pity[[/labelnote]] and try to befriend said animatronics, to varying levels of success.
** And any setting where AIIsACrapshoot and advanced robotics are canon is open to some sort of FusionFic.
* ''VideoGame/Halo3'' ended with Master Chief and Cortana adrift in space, with the former going into [[HumanPopsicle cryosleep]] until rescue could arrive. Many fanfics have the remains of the ''Forward Unto Dawn'' crash landing or being found in any number of other settings. Alongside that there's also the malfunctioned slipspace portal at the end of the game as well.
* ''VideoGame/KerbalSpaceProgram'' is a {{Construction and Management Game|s}} about space exploration. Many fan works and more than one GameMod has focused on the possibility of the Kerbals moving beyond their own solar system, which leaves almost infinite scope for crossovers.
* ''Franchise/KingdomHearts'', given the nature of its cosmology, can have any world be a part of the larger multiverse. There's an alternate method for ''Franchise/FinalFantasy'' characters, as well, as they can show up as a resident of one of the series's original worlds (as seen with the inhabitants of Destiny Islands and Hollow Bastion/Radiant Garden). If that wasn't enough, ''[[VideoGame/KingdomHeartsII II]]'' strongly implies that its depiction of [[VideoGame/FinalFantasyX Auron]] is the real deal and ''not'' some AU version created for the series [[labelnote:(Spoilers!)]]When destroyed, the statue [[WesternAnimation/{{Hercules}} Hades]] used to control Auron releases memories belonging to Auron, which plays audio from his pilgrimage with Braska and Jecht prior to the events of ''FFX''. Furthermore, Auron leaves behind pyreflies -- a phenomenon exclusive to Spira -- when vanishing from the Olympus Coliseum during the credits.[[/labelnote]], while ''[[VideoGame/KingdomHearts3DDreamDropDistance 3D]]'' featured the cast of ''VideoGame/TheWorldEndsWithYou'', who are likewise the actual characters from their game of origin.
* ''Franchise/MassEffect'' often has the canon Systems Alliance [[FusionFic replaced with the government of humanity from another fiction]]. Or has an alien/non-human race from another work activate a Mass Relay.
* ''VideoGame/Metro2033'' has the laws of physics breaking down at times, in what is known as "anomalies". Which fanfic writers often use for dropping characters from other franchises into the Metro, and vice versa.
* ''Franchise/MortalKombat'':
** The mythos holds that "Your world is but one of many realms," allowing the title tournament to be held in pretty much any world.
** ''VideoGame/MortalKombat11'' gives us Kronika's Hourglass, which can bring in characters from other timelines, and is implied to be the explanation for the guest characters' appearances.
* The D'ni of ''VideoGame/{{Myst}}'' can link anywhere. So your whoever is just strolling along when suddenly there's a guy with a book in front of them...
* This is taken to new extremes in ''VideoGame/ProjectXZone'', a crossover of dozens of incompatible universes. It's accomplished by various time-space anomalies which, by necessity, make basically no sense, but which the game ''will not shut up about''. As ''Project X Zone'' is a sequel to ''VideoGame/NamcoXCapcom'' and ''VideoGame/EndlessFrontier'', both plot-wise and {{spiritual|Successor}}ly, and these games abound with {{Mythology Gag}}s to other previous crossovers (for instance, [[VideoGame/FightingVipers Bahn]] is already familiar with [[VideoGame/VirtuaFighter Akira and Pai]] because they all appeared in ''VideoGame/FightersMegamix'' together), which in turn hint at even more titles being connected in some form (see the IntercontinuityCrossover page for examples), one could posit that the majority of Creator/{{Capcom}}, [[Creator/BandaiNamcoEntertainment Namco]], Creator/{{Sega}}'s, and Creator/{{Nintendo}}'s franchises exist in the same universe (or as part of a greater multiverse) and could logically cross over with one another.
* ''Franchise/RatchetAndClank'':
** The titular duo crash-landing (or normal-landing) on the planet the other work occurs on is a relatively common one.
* ''Franchise/SonicTheHedgehog'':
** The [[VideoGame/SonicStorybookSeries Storybook Series]] features Sonic being [[PortalBook sucked into a storybook world]] where [[UniversalAdaptorCast everyone looks suspiciously like his friends]], except for one woman with purple hair and the male villain. Fanfics often feature Sonic venturing into other classic novels or myths, or occasionally even more contemporary pieces. Whether everyone resembles his friends, or whether the purple-haired woman or male villain appear, varies.
** Crossing over with other series isn't all that uncommon either, with a common method for it being an accident involving Chaos Control (or really, just the Chaos Emeralds in general), a la the beginning of ''Anime/SonicX'' (which had Sonic and friends transported to Earth through a freak accident involving one of Eggman's machines, triggering Chaos Control).
* ''Franchise/SuperMarioBros'':
** Warp Pipes. They can lead ANYWHERE, even to other dimensions.
** The Void from ''VideoGame/SuperPaperMario''. It threatens all worlds, so that can easily translate to "whatever fictional world the author wants to tie in with it." Same with the powers to travel dimensions/alternate universes.
* ''VideoGame/SuperRobotWars'':
** Aside from the aforementioned [[Anime/SuperDimensionCenturyOrguss Space-Time Bomb]] in [[VideoGame/SuperRobotWarsZ Z]], the sequel, ''Super Robot Wars Z2'', introduces a seemingly-natural phonomenon called Dimensional Quakes [[spoiler:which was actually caused by the Bomb in the previous game]]. It's because of these Quakes that this game's world has [[Anime/CodeGeass a second Japan]], [[Anime/TengenToppaGurrenLagann a second moon and a new continent]] among other things.
** In [[VideoGame/SuperRobotWarsX Super Robot Wars X]], the Keepers of Order have the power to summon "otherworlders" to the world of Al-Warth. Who knows who or what the Keepers could summon next?
* ''VideoGame/SuperSmashBros'', even when it [[ExcusePlot has a plot]], never really bothers with the "why" and the "how" of all these video game characters being together, so anybody can be thrown in for any reason, but the fan-favorite method is having the preferred character(s) getting an invitation from Master Hand to join his "fighting tournament". This was popular long before some character trailers in ''[[VideoGame/SuperSmashBrosForNintendo3DSAndWiiU Smash 4]]'' onwards started using it officially.
* ''Franchise/TouhouProject'''s Yukari Yakumo has been known to "spirit away" unsuspecting individuals from outside who she takes an interest in and take them to Gensokyo.
* ''VideoGame/TownOfSalem'' fanfics where the cast of any franchise sit together and play a round of ''Town of Salem'' while staying fully in character in their decision making have started popping up. As ''Town of Salem'' is a simple party game, it's easy to just say someone in the setting came up with the rule set and made it into an app (for settings with technology) or using magic to make sure the game is played correctly.
* One classic in ''VideoGame/{{Undertale}}'' is to replace the Human Child falling into the Underground with a different character of your choice. A few examples are [[VideoGame/{{LISA}} Terry]] [[http://thegentlecoltteofleisure.tumblr.com/post/132487105015/shitty-fanfic-corner Hintz]], [[VideoGame/{{LISA}} Bud]][[http://archiveofourown.org/works/5383121/chapters/12433178 dy]], [[WesternAnimation/GravityFalls The Pi]][[http://archiveofourown.org/works/5241128/chapters/12090290 ne Twins]], [[WebAnimation/{{RWBY}} Ruby]] [[https://www.fanfiction.net/s/11773211/1/ Rose]], [[VideoGame/{{Iji}} I]][[https://www.fanfiction.net/s/11793486/1/ ji]], [[Manga/{{Naruto}} Nar]][[https://www.fanfiction.net/s/11594766/1/ uto]], [[https://discount-supervillain.tumblr.com/post/144439159537 Jasper]] from ''Franchise/StevenUniverse'' (collectively known as [[https://media.tumblr.com/4c76ff3044710646d1d689153d0612dd/tumblr_inline_o76mdveZpr1r5xwon_1280.png Jaspertale]]), [[VideoGame/Mother3 and even Lucas]] [[https://www.fanfiction.net/s/11949375/1/ and company.]]
* The portals in ''VideoGame/TheWitcher3WildHunt'' are apparently a way for Geralt to "accidentally" end up in another universe instead as a GuestFighter, a plot device that would be officially used in ''VideoGame/SoulcaliburVI'' and ''VideoGame/MonsterHunterWorld''. Although, it's probably justified because the portals will throw Geralt off-target as seen in-game.
[[/folder]]

[[folder:Visual Novels]]
* With the ''Franchise/AceAttorney'' series, all you need to do is have Phoenix be hired by any fictional characters of your choosing, so he can get their friend off of a murder charge. Since ''Ace Attorney'' has plenty of strange people and ambiguously magical elements (the channeling, Apollo's ability, etc), writers are really only limited by their ability to write a good turnabout mystery. This is so common, it's Phoenix's excuse for showing up in ''VideoGame/ProjectXZone 2'': He's representing [[Franchise/{{Tekken}} Heihachi Mishima]] on charges of starting a ZombieApocalypse, and tags along with the party because the bad guys who framed his client figure that Heihachi's lawyer would be an easier target than Heihachi himself. He also previously successfully defended [[VideoGame/LikeADragon Goro Majima]] on some minor charge.
* For ''Franchise/DanganRonpa'', take a bunch of characters from other works of media, and use their talents to turn them into Ultimate students. From there, they have an easy excuse for them to get involved in a killing game.
* ''Franchise/{{Nasuverse}}'':
** The entire concept of Servants from ''Franchise/FateSeries'' means that it's easy to summon a crossover character as a Servant. There are even canonical examples of fictional characters being summoned, so they don't even need to be established as previously existing in the same universe. Likewise, characters from other settings starting their own Grail Wars.
** The Nasuverse as a whole has Zelretch, a [[DimensionalTraveler dimension-hopping]] sorcerer with [[ItAmusedMe a weird sense of humor]] who makes minor appearances in both ''VisualNovel/FateStayNight'' and ''VisualNovel/{{Tsukihime}}''.
[[/folder]]

[[folder:Webcomics]]
* The game of SBURB from ''Webcomic/{{Homestuck}}'' assembles a group of adolescents from one universe and brings them to Skaia, which is ''[[BroadStrokes mostly]]'' the same in each game -- which means it's easy to have crossover characters interact with Jack Noir and other natives of Derse or Prospit. Since other universes have interfered with canon sessions, involving multiple non-''Homestuck'' series is also possible in a crossover fic.
* The Mutex in ''Webcomic/GeneralProtectionFault'' can bring the characters to other universes, [[FisherKingdom altering them to fit]]. In canon, this has facilitated an official ''Webcomic/KevinAndKell'' crossover, and pastiches of franchises as varied as ''ComicStrip/KrazyKat'' and ''Franchise/TheLegendOfZelda''.
[[/folder]]

[[folder:Web Original]]
* The basic premise of Weboriginal/TheBackrooms is that [[ParanoiaFuel anywhere and anytime you can "noclip" out of reality]] to find yourself in the titular EldritchLocation. So you can choose any fictional universe and character of your liking, and make them to fight their way throughout the backrooms.
* MediaNotes/JennyEverywhere is defined by the gimmick that there is a version of her in ''every'' reality, so it is part of the premise itself that she supposedly ''already'' exists in whatever fictional universe you want to write about -- ''just'' off-screen... Additionally, most versions of Jenny are {{Dimensional Traveler}}s who can leave their home reality at will to explore TheMultiverse.
* ''WebVideo/PoohsAdventures'':
** The series is pretty much a crossover genre in of itself, with a couple of crossover methods that get used to put Pooh in the setting of the movie. Coincidentally, the most used of these methods are all based from scenes of ''WesternAnimation/TheNewAdventuresOfWinnieThePooh'' episode "Paw and Order."
** [[ShowWithinAShow One idea is that Pooh and his friends are in a play that just happens to be the movie they're to be in]], though [[MindScrew it might be confusing to some people]] [[FridgeLogic and they start asking questions]].
** The gang get lost in the desert and wind up in the middle of somewhere. Usually useful for urban settings, [[PatchworkMap even when it's not feasibly possible]].
* ''WebAnimation/{{RWBY}}'':
** Dust [[MagicByAnyOtherName (the series' version of "magic" propellant)]] tends to be used for a variety of things, some of it implying fast travel or teleportation.
** An ''astounding'' amount of crossover and self-insert fics usually start with the crossover characters just magically appearing in the Emerald Forest, conveniently in groups of four and [[FountainOfYouth sometimes physically changed to the ripe age of 17]] for the (author's) express purpose of allowing them to attend Beacon Academy, qualifications be damned.
** [[spoiler:With the ending of volume 8 featuring Team RWBY and Jaune falling into the VoidBetweenWorlds, it's relatively easy to have them land anywhere you could want as a result.]]
* A popular ''Website/SCPFoundation'' fanfic theme is an SCP report on crossover anomalies such as the Manga/DeathNote or [[Franchise/TouhouProject Gensokyo]].
* Many times, the Red and Blue Teams (of ''WebAnimation/RedVsBlue'' fame) are explicitly AI replicas of the main cast, except for Church (who [[BrainUploading uploaded his mind into a computer]]) in a Virtual Reality sim of a fictional setting run on the same computer.
* ''WebVideo/BarneyBunch'':
** The basic idea of the videos are pretty much a crossover concept in and of itself. It's possible to choose any fictional (and preferably male) character of your liking and turn them into a extremely promiscuous homosexual who regularly does unspeakable things.
** A large number of "Drew Pickles Goes to" videos where he travels into the worlds of TV shows and movies usually have him get there by popping in a VHS or DVD of the show or movie in question [[TelevisionPortal and then jumping into the TV screen]].
[[/folder]]

[[folder:Western Animation]]
* If it's [[MouseWorld mousepunk]], it's pretty much accepted that it shares the same universe with any other work out there that's [[PunkPunk mousepunk]], too, by default. ''Literature/TheRescuers'', ''WesternAnimation/ChipNDaleRescueRangers'', ''Literature/MrsFrisbyAndTheRatsOfNIMH'', ''WesternAnimation/FlushedAway'', ''WesternAnimation/DangerMouse'' (if Danger Mouse and Penfold are declared rodent-sized, that is) and so forth, even ''WesternAnimation/TheGreatMouseDetective'', ''WesternAnimation/AnAmericanTail'' and/or ''Literature/{{Redwall}}'' if {{Flashback}}s, TimeTravel or historical artifacts are involved. Of course, that doesn't mean that mousepunk cannot be crossed over with anything else; in fact, that's quite easy, too, as the vast majority of works in this genre already has the rodent society existing alongside humans and going unnoticed already. ''Redwall'' is an exception, as no humans appear in it, but it's commonly thought of as taking place in the far future relative to the other works in this "universe".
* ''WesternAnimation/DannyPhantom'' has all of the unnamed doors in the Ghost Zone. Who's to say that one of them doesn't open to a different fictional universe? There's also the naturally-occurring portals into the Ghost Zone that canonically transverse space-time, which can be used for any number of crossovers.
* ''WesternAnimation/{{Daria}}'' has a BizarroEpisode where [[AnthropomorphicPersonification holiday spirits]] come to town through an interdimensional wormhole behind a Chinese restaurant. Despite being FanonDiscontinuity for the longest time, eventually people started to wonder: what if that wormhole was real, and it could take you to ''all sorts'' of other worlds?
* ''WesternAnimation/TheFairlyOddParents'': Considering Timmy has two RealityWarper Godparents who will grant his every wish, most crossovers end up being as simple to write as 'Timmy wishes he was in this fictional world'. It's to the point where [[WesternAnimation/TheJimmyTimmyPowerHour even the show itself used this]] to [[WesternAnimation/TheAdventuresOfJimmyNeutronBoyGenius facilitate a crossover with a fellow Nicktoon]].
* ''WesternAnimation/HazbinHotel'': Given that the show takers place in Hell, one can simply have a charcter die and go to Hell. Alternatively, have them be a target for [[WebAnimation/HelluvaBoss I.M.P.]].
* ''WesternAnimation/{{Hilda}}'': Near the end of season 1, Hilda learns about Nowhere Space; a PocketDimension that exists in every building. She is warned by the Nisse that the outside world also has a Nowhere Space, but due to its immense size one could get lost inside forever, or end up at any point in the world at random. ''Any'' point.
* ''WesternAnimation/InfinityTrain'' has the titular train, which appears in front of anyone who is experiencing severe emotional/psychological distress at a pivotal moment in their lives. So long as your chosen character has some personal issues they need to work out, you have a perfect excuse for them to unknowingly board a supernatural train filled with an unquantifiable number of pocket universes.
* ''WesternAnimation/InsideOut'' is a rather extreme case, as you don't even need to feature any of the film's characters, just its concept. Pick any character from any story you want and show their versions of Joy, Sadness, Disgust, Anger and Fear, plus their core memories and islands.
* There are a number of ''WesternAnimation/InvaderZim'' fics based on the premise of Zim landing elsewhere on Earth than where he does canonically, often being used as an excuse for crossovers.
* ''WesternAnimation/KimPossible'' features in two episodes the Pan-Dimensional Vortex Inducer which, when not threatening to rip a Nevada-sized hole in the fabric of space-time, can be used to send characters to other dimensions... including inside TV shows (as the show itself demonstrated). There are enough {{Mad Scientist}}s around to experiment with that type of technology.
* ''WesternAnimation/MyLittlePonyFriendshipIsMagic'':
** While it goes unmentioned in the main series, there is canonically a magic mirror that acts as a portal to the ''WesternAnimation/MyLittlePonyEquestriaGirls'' HighSchoolAU, while the [[LooseCanon loosely canonical]] [[ComicBook/MyLittlePonyFriendshipIsMagicIDW comic series]] established a different magic mirror that links to a MirrorUniverse. Who knows what other mirrors may be out there somewhere?
*** A HighSchoolAU is already enough to establish reasonably easy crossovers with many series featuring OrdinaryHighSchoolStudent, and if you're willing to invoke a SugarApocalypse such as an AlienInvasion, [[Fanfic/{{Ranger}} well sky's no longer the limit]]...
** Discord can open portals to other dimensions, including a canonical live action one, and is a FourthWallObserver and complete {{Troll}} who'd definitely set up a crossover for fun, especially if the story is set before his HeelFaceTurn.
** Several characters, particularly Twilight Sparkle and Starlight Glimmer, display incredibly strong magical powers which include things like teleportation, time travel, and other reality-bending abilities. They also sometimes are incapable of completely controlling these powers. Thus, an extremely common crossover plot is "Twilight or someone else screws up (or is distracted while casting) a spell", leading to an individual or group from either Equestria or the crossover series winding up in the others' world.
* The premise of ''WesternAnimation/PennZeroPartTimeHero'' involves the main characters traveling to different dimensions, taking the places of people in those dimensions, and helping to save the day. It's easy to imagine a crossover with any other series through this; just have Penn and his friends temporarily take over the bodies of three characters (and Rippen take over the body of the main villain) and you're good to go.
* ''WesternAnimation/{{Pibby}}'': The short's status as already being a crossover, with the potential for anything connecting to it, alongside the crossover-friendly premise of "the Corruption follows Pibby to a new world and she has to team up with surviving natives to fight it", has resulted in numerous fanart depicting Pibby traveling to almost every piece of fiction people can think of, mainly of the animated kind (whether western cartoon, anime, web animation, etc.).
* ''WesternAnimation/ReBoot'' can cross over with any electronic game capable of being run on a PC, or if need be, a ''supercomputer''. That includes {{licensed game}}s (or at least licensed PC programs, like activity center programs) of properties primarily known for another medium. The work you want to use has no such game based on it? Just say it does and get to doing what you want!
* ''WesternAnimation/RickAndMorty'':
** Rick has a device that can open a portal to any of infinitely many universes, similar to the ''WesternAnimation/StarVsTheForcesOfEvil'' example listed below.
** This show and ''WesternAnimation/GravityFalls'' have put numerous [[LawyerFriendlyCameo nods]] at each other, and both involve elderly {{Dimensional Traveler}}s. At this point, it's basically accepted by the fandom that Rick and Ford have run into each other at least once during their adventures.
* The final season of ''WesternAnimation/TheSmurfs1981'' involved a format of the Smurfs getting transported to different time periods by way of a magical "time whirlwind". One could initiate a crossover with any series by having the time whirlwind drop the Smurfs off in that series' universe or the time period that series takes place in.
* ''WesternAnimation/StarVsTheForcesOfEvil'' features dimensional scissors, which allow characters to cut into alternate dimensions. One could initiate a crossover with any series just by having Star snip her way into that world.
* ''WesternAnimation/TeenageMutantNinjaTurtles2012'': After the Manhattan Project episode, expect to see a ton of Kraang portal-based crossovers.
* ''WesternAnimation/TeenTitans2003'' has the simple fact that the team is in the DC Comics universe. The most common crossover is Robin having to go back to Gotham or Batman/Batgirl coming to him, or something involving his future alter ego Nightwing.
* ''WesternAnimation/TotalDrama'' has "What if [teenage character from another franchise] was a contestant?"
* ''Franchise/{{Transformers}}'':
** The franchise has the obvious solution of simply dropping the Ark onto the other setting, with Cybertron being uncounted light-years away from Earth and the entire cast being packaged and delivered straight a new setting.
** It's equally easy for a Transformer to disguise themselves as a vehicle from whatever franchise they're crossing over with.
** It's easy to initiate a crossover with a different series by having the Transformers transported to that series' world by way of a malfunctioning [[PortalNetwork space bridge]]. The ''[[WesternAnimation/MyLittlePonyFriendshipIsMagic My Little Pony]]'' [[ComicBook/MyLittlePonyTransformersFriendshipInDisguise crossover]] is an official use of this method.
[[/folder]]
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