Some crossovers start a storyline in one piece of media and finish it in another. Others bring existing characters to the same place. A crossover alternate universe is where a universe is created solely for the purpose of a bringing together characters for whom the above two options aren't available.
This usually happens for one of the following three reasons:
1. A fake crossover is needed. The writers can't simply transport all the characters to an existing universe due to plot necessities or copyright reasons.
2. The original work is not ongoing. It would be impossible to have the characters pop up in existing stories. This is common in Fairy Tale Free For Alls and other works involving public domain characters.
3. The writers need to give the characters backstories different from what they are in their original stories.
Usually (but not always), the universe isn't used again after the crossover finishes. Subtrope of Alternate Universe.
- This technique is used whenever a crossover happens between Marvel Comics and DC Comics. Most notable is the Amalgam Universe.
- Kill Shakespeare takes place in a universe in which the characters' experiences differ from all of their backstories in the original works.
- Played with in Spider-Man: Into the Spider-Verse. The universe is separate from any existing continuity (it was created for the movie) and doesn't initially appear to be a crossover, but that changes about 40 minutes into the film when the other Spider-Men are brought in from other universes.
- Hikonin Sentai Akibaranger, the main characters can create a delusional universe where every Super Sentai team exists.
- Fate/Grand Order, the Crisis Crossover for the Fate series, takes place in its own timeline separate from other works. Played with in that the franchise is known for having Alternate Universes being a known fact in-universe, and that the characters from the other Fate works are explicitly pulled from there with the special Summoning Ritual unique to Grand Order's world or via special crossovers.
- Marvel vs. Capcom series take place in a separate Marvel universe called Earth-96169.
- Injustice: Gods Among Us and its sequel both take place in one of the DC lore's many unaccounted for alternate universes.
- The Project X Zone games take place in a crossover world inhabited by Original Generation characters that has a red-hot go at being a crossover of crossovers between Capcom, Namco, and Sega. If disparate characters have ever shared a spotlight before (i.e. Street Fighter X Tekken, Marvel vs. Capcom), expect them to recognize each other here. However, PXZ and its sequel are explicitly a Post-Script Season to each and every one of its sources to avoid plot conflicts, while several plot threads in both games are a direct continuation of the events of Namco × Capcom (and, to a lesser extent, Endless Frontier).
- Super Smash Bros., particularly the story modes in Brawl and Ultimate, establish a universe where all the Nintendo characters (plus Snake, Cloud, the Belmonts, etc.) co-exist in the same universe. Even more extreme in Ultimate, where The Legend of Zelda and Castlevania characters are drawn from different points in their own continuities' timeline. note While it was hinted before, in one of Ultimate's pre-release streams, Sakurai confirmed that the characters in the Smash Bros. games are toys in the realm of imagination.
- According to Eiji Aonuma, Hyrule Warriors takes place in an separate dimension from the canonical games of The Legend of Zelda, and thus does not fall within any of the three timelines revealed in Hyrule Historia. There's a Link, Zelda, Impa, and Ganondorf in this dimension (given the Legacy Character nature of those four), but all other returning Zelda characters (such as Midna, Darunia, and Fi) are pulled from portals into other timelines in order to make this crossover possible.
- Shonen Jump crossover games:
- Rakenzarn Tales makes its own world called Rakenzarn into order to bring together bunches of existing characters from varying series.
- Kingdom Hearts takes place in a universe where Disney and Square Enix universes collide. Specifically, the implication is that nearly every entry of the Disney Animated Canon, most Pixar films, films and features falling outside of either category (ex. The Nightmare Before Christmas, Mickey, Donald, Goofy: The Three Musketeers), and even live-action movies like TRON and Pirates of the Caribbean are connected on the Disney side. Square's side of the crossover is currently more limited, mainly consisting of AU versions of various Final Fantasy characters (with the possible exception of Auron in Kingdom Hearts II, hinted to be a younger version pulled from a point in time prior to FFX). Additionally, the cast of The World Ends with You play a supporting role in Dream Drop Distance and elements from Einhänder made their way into the Gummi Ship segments of Kingdom Hearts III (including two enemies appearing as Bonus Bosses).