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Obvious Crossover Method

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Some series formats (Doctor Who, Quantum Leap) lend themselves particularly well to crossovers, since the primary texts involve a constant dislocation of the protagonists. The TARDIS has materialized everywhere from the Planet of the Apes to Fawlty Towers, even on the set of Wheel of Fortune. Sam Beckett finds himself occupying the bodies of everything from Martian invaders (The War of the Worlds) to Indiana Jones 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 fanfic. Sure, it's easy to do a crossover when the characters could feasibly come from the same world (often modern Earth), but what about when they simply couldn't, such as each existing in a distinct Constructed World? 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 Fandom-Specific Plot, 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 Science Fiction/Science Fantasy genres, thanks to the abundance of Alternate Universes, wormholes, Negative Space Wedgies, and big galactic settings where the other works could be said to be hidden away in some other corner of the universe. A Mutual Masquerade is another useful plot device, as are works with an Intrepid Fictioneer premise.

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

Compare Sailor Earth (a Sister Trope of a gap in the setting that can be filled with an Original Character, a minor character, or, overlapping with this, a crossover character), The Stations of the Canon (which is more about a frequent side effect; this is about a possible cause).


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    Anime & Manga 
  • Ah! My Goddess 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.
  • Digimon: It is established in Digimon Adventure 02 that multiple universes exist, as well as multiple parallel Digital Worlds of the other various Digimon animes, with the various characters managing to 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 Dragon Ball Z 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 The Familiar of Zero to summon a crossover character is the basis of the majority of the series's fanfics in both English and Japanese. There is a Japanese wiki dedicated to compiling the fanfics from 2ch's thread on replacing Saito with other characters.
  • The Gate from Fullmetal Alchemist (2003). We know it leads to one parallel universe, so why not another?
  • Instead of the eponymous Gate opening up in modern-day Tokyo and the Empire attempting to invade that it can literally open up anywhere else.
  • Hell Girl 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 massive crossover list on FanFiction.Net.
  • Because the characters of Hetalia: Axis Powers 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.
  • Neon Genesis Evangelion:
    • Fanfiction often has Shinji Ikari being raised by crossover characters (instead of his unnamed caretakernote ) before being called back by Gendo.
    • Being pulled through the Dirac Sea to another (crossover) universe during the fight with Leliel.
  • Zoro of One Piece is a frequent source of Memetic Mutation in regards to his infamous No Sense of Direction, namely writers keep putting him in many other franchises (not just anime, but films, books, even in real life).
  • Ranma ½:
    • The Nanban Mirror from a filler episode allows Time Travel 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.
    • Revisioning Ranma's mother, Nodokanote  as related to characters from the crossover series, which gets revealed in time for the plot to kick in.
    • Also common: Ryōga 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].
  • Super Dimension Fortress Macross (and/or 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 Urusei Yatsura where Ataru is replaced as Lum's opponent in the tag game (and accidental fiancé) with a teenaged character from a different fandom.
  • Pretty much every work has at least one crossover fic with ×××HOLiC where a character will come to Yuuko for a wish. The character's ability to come to her shop is usually explained by "Hitsuzen."
  • The Space-Time Oscillation Bomb from Super Dimension Century Orguss, 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 Super Robot Wars games, Super Robot Wars Z, 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 Kengan Ashura by have them participate in a Kengan Associacion match or tournament. In fact, this is how creators themselves set up crossover one-shots with Street Fighter and Baki the Grappler. 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.

    Comic Books 
  • The Multiverses of DC and Marvel, along with lesser-known examples like WildStorm's "Snowflake," are easy to stretch to include other crossover settings. (It's kind of canonical that the Marvel and DC multiverses include each other.)
  • Power rings. They can fly to any character from space and recruit them, tends to cling onto the chosen whether they want it or not, and is attracted to powerful quantities of common qualities among fictional characters. For example:
    "Zuko of the Fire Nation. You have great rage in your heart. You belong to the Red Lantern Corps."
    "Charles Burns of Springfield. You want it all."
    "Zaros of Freneskae. You have the ability to instill great fear. Welcome to the Sinestro Corps."
    "Theodore Roosevelt of Earth. You have the ability to overcome great fear. Welcome to the Green Lantern Corps."
    "Luke Skywalker of Tatooine. You have the ability to instill great hope. Welcome to the Blue Lantern Corps."
    "Braum of Runeterra. You have the ability to feel great compassion. Welcome to the Indigo Tribe."
    "Anna of Arendelle. You have great love in your heart. Welcome to the Star Sapphires."
  • Mickey Mouse: 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 The Sandman (1989) 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.
  • Wolverine:
    • The biography of Wolverine easily allows characters from other fandoms to be his descendants, often ones he is unaware of. The comics already canonically did this with the introduction of 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.

    Fan Works 
  • The Infinite Loops: Half the point of the setting is to provide easy crossovers. 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.

  • The basic concept of how dimension-hopping works in Everything Everywhere All at Once 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 Final Destination 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 nature, powers and already having a few official crossovers, it's not uncommon for the iconic killer of the A Nightmare on Elm Street franchise to use his abilities to pop up in other franchises and menace certain characters in their dreams.
  • Star Wars 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 did it at one point as an April Fools joke, adding entries for characters from Willow written as if they lived on the planet Andowyne in the Star Wars galaxy.
  • Stay Tuned has great crossover potential, as Hellvision has 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.

  • 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 Time Matrix transports the Animorphs to another universe.
  • 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.
  • Harry Potter:
    • 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 Harry Potter and the Order of the Phoenix, 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 fundamentally incompatible with Harry Potter's.
    • For a series with teen protagonists, they attend Hogwarts as "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 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.
  • 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 The Incarnae, 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 Word of God suggests that Pact, Twig, and the various snippets written by Wildbow exist elsewhere in the Worm multiverse.
    • One Fandom-Specific Plot 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.

    Live-Action TV 
  • Battlestar Galactica (2003) 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.
  • The Big Bang Theory: Several crossover stories involve characters from other franchises getting jobs at Caltech, often working as lab assistants to the protagonists.
  • The witches of Bewitched can manifest fictional characters from any continuity and transport themselves into fictional realms.
  • The Black Mirror episode "San Junipero" features 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.
  • Buffy the Vampire Slayer has at least five:
    • "YAHF" (Yet Another Halloween Fic) — the many, many stories based on the second season Halloween episode where a spell makes everyone Become the Costume.note  Naturally, a crossover is as simple as changing which costumes The Little Shop That Wasn't There Yesterday 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 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 Brainwash Residue 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 "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.
  • Jensen Ackles played two characters on Dark Angel a few years before playing Dean Winchester on Supernatural, Serial Killer Ben and Lovable Rogue Alec, who were genetically engineered twin clones, which naturally lends itself to crossovers where Dean was the source of their DNA.
  • Dark Matter (2015) 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/Stargate crossover using this conceit came very close to materializing in canon when MGM's Stargate Command streaming service was launched (both franchises had Joseph Mallozzi and his longtime writing partner Paul Mullie as showrunners).
  • Since Doctor Who 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 Alternate Universe due to a glitch, or else the Timey-Wimey Ball 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: 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 Iris Wildthyme, who originated outside Doctor Who continuity before being transplanted into the Doctor's Expanded Universe and crosses continuities with ease in her smaller-on-the-inside Celestial Omnibus.
  • Season 2 of The Flash (2014) 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 connect the 'verse to previously unrelated stories like Supergirl.
  • Farscape. Once Crichton figures out wormholes, and wormholes are revealed to possibly lead to alternate universes, pretty much anything is fair game.
  • Fringe has alternate world travel in the later seasons.
  • 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 Immortal Procreation Clause). 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.
  • Kamen Rider Decade has the plot of "Tsukasa and his friends travel from world 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 lacked any Kamen Riders at all. And considering how their travels are random and there's an And the Adventure Continues moment in the Grand Finale...
  • Given the nature of Murder, She Wrote, 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.
  • Once Upon a Time 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 Agrabah to Arendelle to Camelot to 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.
  • Quantum Leap runs on Hand Wave 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 Sliders simply drops the characters in the crossover world.
  • Stargate:
    • 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 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.
  • Star Trek is so full of Negative Space Wedgies, Swirly Energy Thingies, Teleporter Accidents, Holodeck Malfunctions, and other technobabble that it's practically a grab bag for crossovers. Plus the Sufficiently Advanced Aliens. For example, the Star Wars crossover Conquest used a wormhole, with Q constantly implying he had a hand in that.
  • Supernatural can have an angel or a supernatural object transport characters into other works. Examples of both, leading to Trapped in TV Land scenarios such as the Scooby-Doo crossover episode "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 referenced SPN as fictional less of an issue.

    Tabletop Games 
  • Dungeons & Dragons:
    • 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 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.
    • 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.
    • 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 The Fair Folk 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 Krynn or characters from, normally cut-off from rest of the Multiverse, world of Mystara make a cameo in The Wild Beyond the Witchlight. 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 GURPS setting Infinite Worlds 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).
  • Magic: The Gathering 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 Rifts was practically built for crossovers — one never knows where a Rift might lead.
  • TORG was also built for crossovers, since the main premise of the game is that alternate universes are "invading our world" and overwriting it.
  • Warhammer 40,000 crossovers usually involve a mishap with the warp. Also, there are stated to be gaps between the systems of the Imperium where a small to medium-sized (human or not) civilisation can fit in. Sometimes, a Primarch's pod (maybe one of the lost Primarchs) happens to land on a planet.

    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 Assassin's Creed were in the same place at the same time. You can have that one for free, fan-fiction writers."
  • BioShock Infinite, and by extension the rest of the series, thanks to the infinite worlds shown at the end of the game.
  • Dissidia Final Fantasy: 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 NT, you can just slot any new faces into that game's conflict, as it's a Distant Sequel to the original conflict between Cosmos and Chaos (only with their successors taking up the mantle) wherein the Final Fantasy characters are usually pulled from a point both after their canonical journeys and the thirteenth cycle.
  • Doom (2016) 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 Doom Eternal. Several crossovers start with the pair being teleported away and then landing on another planet or universe.
  • The Elder Scrolls V: Skyrim: The "Toddroll" meme ("Hey, you, you're finally awake.") started by shitposting Fallout 76 as a Stealth Sequel, and paved the way to popularize the idea of characters from any franchise waking up in the cart from the start of Skyrim after being knocked unconscious for any reason.
  • Fallout: New Vegas:
    • The Big Mountain Transportalponder from 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.
      • 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.
  • Fire Emblem: Awakening has the Outrealm Gate, which can transport the characters to other worlds and alternate timelines. This is to the extent of being an 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 the series. Fire Emblem Fates would expand upon this concept (particularly since three of the Nohrian characters are returning second generation units from Awakening under new aliases, having crossed over at the behest of the Player Character's father), and later games and media (Heroes, Cipher, Warriors, etc.) implicitly make use of Outrealm Gates to justify previously established characters popping up in an Original Generation setting (or, in the case of two DLC maps for Shadows of Valentia, the exact opposite). This is even canonical within the greater Nintendo library of IPs, as Robin, Lucina, and Corrin's appearances in the fourth Super Smash Bros. are officially Alternate Selves of those characters (with Corrin's reveal trailer having him take a fourth option during the Branch of Fate that would normally lead to Birthright, Conquest, or Revelation and decide to join Smash instead).
  • Five Nights at Freddy's has at least two of these:
    • First, there's the use of other Suck E. Cheese's (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 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). Dante's Night at Freddy's is one such example. In a more lighthearted and friendly note, some can find out what is going on with said animatronics and try to befriend said animatronics, to varying levels of success.
    • And any setting where A.I. Is a Crapshoot and advanced robotics are canon is open to some sort of Fusion Fic.
  • Halo 3 ended with Master Chief and Cortana adrift in space, with the former going into 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.
  • Kerbal Space Program is a Construction and Management Game about space exploration. Many fan works and more than one Game Mod has focused on the possibility of the Kerbals moving beyond their own solar system, which leaves almost infinite scope for crossovers.
  • Kingdom Hearts, given the nature of its cosmology, can have any world be a part of the larger multiverse. There's an alternate method for Final Fantasy 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, II strongly implies that its depiction of Auron is the real deal and not some AU version created for the series (Spoilers!) , while 3D featured the cast of The World Ends with You, who are likewise the actual characters from their game of origin.
  • Mass Effect often has the canon Systems Alliance replaced with the government of humanity from another fiction. Or has an alien/non-human race from another work activate a Mass Relay.
  • Metro 2033 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.
  • Mortal Kombat:
    • The mythos holds that "Your world is but one of many realms," allowing the title tournament to be held in pretty much any world.
    • Mortal Kombat 11 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 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 Project × Zone, 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 Namco × Capcom and Endless Frontier, both plot-wise and spiritually, and these games abound with Mythology Gags to other previous crossovers (for instance, Bahn is already familiar with Akira and Pai because they all appeared in Fighters Megamix together), which in turn hint at even more titles being connected in some form (see the Intercontinuity Crossover page for examples), one could posit that the majority of Capcom, Namco, Sega's, and Nintendo's franchises exist in the same universe (or as part of a greater multiverse) and could logically cross over with one another.
  • Ratchet & Clank:
    • The titular duo crash-landing (or normal-landing) on the planet the other work occurs on is a relatively common one.
  • Sonic the Hedgehog:
    • The Storybook Series features Sonic being sucked into a storybook world where 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 Sonic X (which had Sonic and friends transported to Earth through a freak accident involving one of Eggman's machines, triggering Chaos Control).
  • Super Mario Bros.:
    • Warp Pipes. They can lead ANYWHERE, even to other dimensions.
    • The Void from Super Paper Mario. 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.
  • Super Robot Wars:
    • Aside from the aforementioned Space-Time Bomb in Z, the sequel, Super Robot Wars Z2, introduces a seemingly-natural phonomenon called Dimensional Quakes which was actually caused by the Bomb in the previous game. It's because of these Quakes that this game's world has a second Japan, a second moon and a new continent among other things.
    • In 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?
  • Super Smash Bros., even when it 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 Smash 4 onwards started using it officially.
  • Touhou Project's Yukari Yakumo has been known to "spirit away" unsuspecting individuals from outside who she takes an interest in and take them to Gensokyo.
  • Town of Salem 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 Undertale is to replace the Human Child falling into the Underground with a different character of your choice. A few examples are Terry Hintz, Buddy, The Pine Twins, Ruby Rose, Iji, Naruto, Jasper from Steven Universe (collectively known as Jaspertale), and even Lucas and company.
  • The portals in The Witcher 3: Wild Hunt are apparently a way for Geralt to "accidentally" end up in another universe instead as a Guest Fighter, a plot device that would be officially used in Soulcalibur VI and Monster Hunter: World. Although, it's probably justified because the portals will throw Geralt off-target as seen in-game.

    Visual Novels 
  • With the Ace Attorney 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 Project × Zone 2: He's representing Heihachi Mishima on charges of starting a Zombie Apocalypse, 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 Goro Majima on some minor charge.
  • For 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.
  • Nasuverse:
    • The entire concept of Servants from Fate Series 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 dimension-hopping sorcerer with a weird sense of humor who makes minor appearances in both Fate/stay night and Tsukihime.

  • The game of SBURB from Homestuck assembles a group of adolescents from one universe and brings them to Skaia, which is 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 General Protection Fault can bring the characters to other universes, altering them to fit. In canon, this has facilitated an official Kevin & Kell crossover, and pastiches of franchises as varied as Krazy Kat and The Legend of Zelda.

    Web Original 

    Western Animation 
  • If it's mousepunk, it's pretty much accepted that it shares the same universe with any other work out there that's mousepunk, too, by default. The Rescuers, Chip 'n Dale: Rescue Rangers, Mrs. Frisby and the Rats of NIMH, Flushed Away, Danger Mouse (if Danger Mouse and Penfold are declared rodent-sized, that is) and so forth, even The Great Mouse Detective, An American Tail and/or Redwall if Flashbacks, Time Travel 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".
  • Danny Phantom 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.
  • Daria has a Bizarro Episode where holiday spirits come to town through an interdimensional wormhole behind a Chinese restaurant. Despite being Fanon Discontinuity 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?
  • The Fairly OddParents!: Considering Timmy has two Reality Warper 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 even the show itself used this to facilitate a crossover with a fellow Nicktoon.
  • Hilda: Near the end of season 1, Hilda learns about Nowhere Space; a Pocket Dimension 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.
  • Infinity Train 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.
  • Inside Out 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 Invader Zim fics based on the premise of Zim landing elsewhere on Earth than where he does canonically, often being used as an excuse for crossovers.
  • Kim Possible 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 Scientists around to experiment with that type of technology.
  • My Little Pony: Friendship Is Magic:
    • While it goes unmentioned in the main series, there is canonically a magic mirror that acts as a portal to the My Little Pony: Equestria Girls High School AU, while the loosely canonical comic series established a different magic mirror that links to a Mirror Universe. Who knows what other mirrors may be out there somewhere?
    • Discord can open portals to other dimensions, including a canonical live action one, and is a Fourth-Wall Observer and complete Troll who'd definitely set up a crossover for fun, especially if the story is set before his Heel–Face Turn.
    • 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 Penn Zero: Part-Time Hero 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.
  • 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.).
  • ReBoot can cross over with any electronic game capable of being run on a PC, or if need be, a supercomputer. That includes licensed games (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!
  • Rick and Morty:
    • Rick has a device that can open a portal to any of infinitely many universes, similar to the Star vs. the Forces of Evil example listed below.
    • This show and Gravity Falls have put numerous nods at each other, and both involve elderly Dimensional Travelers. 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 The Smurfs (1981) 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.
  • Star vs. the Forces of Evil 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.
  • Teenage Mutant Ninja Turtles (2012): After the Manhattan Project episode, expect to see a ton of Kraang portal-based crossovers.
  • Teen Titans (2003) 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.
  • Total Drama has "What if [teenage character from another franchise] was a contestant?"
  • 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 space bridge. The My Little Pony crossover is an official use of this method.