Yeah, I tend to think of fusions as pretty much interchangeable with crossovers, at least that's the way I've written my two crossovers: a character from one fandom is put in the world of the other character.

However I do distinguish between canon divergence/fork-in-the-road works and true AUs. To use a fandom in common as an example, there are plenty of canon divergent FK fics, particularly with regards to the outcome of the series finale. But I don't consider those true AUs because the setting, concept, and themes have remained the same. Vampires are a thing, Nick is both a vampire and a cop, and he's both looking for a cure for his condition and trying to atone for his past actions. Only the outcome diverges. A true AU would change some of the basis of the canon - a mundane setting, for example, where there are no vampires.