Mega Crossover

In fan fiction, a Mega Crossover is a work that combines characters and concepts from two or more independent works of fiction into an coherent whole. Many consider it a challenge because it unites various settings that may contrast greatly. It is often seen as an excuse to use several characters from a person's favorite books, movies, etc. in the same fictional universe. When done right, mega crossovers can be very impressive. An example is Megacrossover Earth.

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Mega Crossover-TV Tropes & Idioms