Fandoms cross over [*] with others easily and well when they have canonically thin or low barriers to other realities (Dungeon Dimensions, time travel, alternate universes, etc.) or are explicitly metafictional. Thus, Discworld, Princess Tutu, the Stargate series, presumably Doctor Who, and so forth.
Given this, I propose Sandman as the universal crossover fandom. Spans pretty much all of time and space, explicitly metafictional, and if all else fails, you can always have the characters meet one of the Endless for thematic or character-development goodness. What do you think?
[*] Just because I personally hadn't heard the term "fusion" until recently: I'm talking about stories where two sources are put together without fundamental changes to either. This is different from fusions, when elements from one source are taken from that source and put in the context of another. Good crossover fandoms aren't necessarily good fusion fandoms; the good fusions I've read have some worldbuilding characteristic that will be fun to set different characters against. His Dark Materials is probably the easiest example, because hey, daemons, but for purposes of this I will link to "X" by trinityofone (general audiences, 3500 words), which fuses Stargate: Atlantis with X-Men, because it delights me. See also: "The You and Whose Army Job" by liviapenn (general audiences, 3500 words), which crosses over SGA and Leverage, and "The Good Guys" by tielan (general audiences, 5500 words), which fuses them. (As a consequence, I think that fusions are a lot harder, or less rewarding, to read if you don't know both sources, which is not necessarily the case for crossovers.)comment(s) | reply | link