Crossovers when two or more fandoms are blended together into one story usually involve fandoms that have some basis of similarity among them. Similar timeframes, landscapes, themes, or characterizations... nodes of nexus from which blending their realities becomes imaginable. Even if the fandoms themselves are very different, there is one least one point of commonality that makes it not only possible but probable to see them mixed.
Then there are the stories like these... crossovers of fandoms so disparate you would never even conceive they could mix. Sure, you could cheat by making one or the other AU; fusions are the very definition of that. But these crossovers aren't fusions, these may make small changes but the do their best to follow the flow of canon. In the expert hands of the most talented authors, you'd be surprised how reasonably these unconventional fandoms can indeed come together.
Unlikeliest Crossovers That Work Unbelievably Well
( Fic Recs - Unlikely Crossovers )
Then there are the stories like these... crossovers of fandoms so disparate you would never even conceive they could mix. Sure, you could cheat by making one or the other AU; fusions are the very definition of that. But these crossovers aren't fusions, these may make small changes but the do their best to follow the flow of canon. In the expert hands of the most talented authors, you'd be surprised how reasonably these unconventional fandoms can indeed come together.
( Fic Recs - Unlikely Crossovers )