Alternate Universe is a catch all for me. From canon-adjacent to 'total', anything that changes a single detail, even if it is just building on canon (post-canon, for instance), it is an AU.
Fusion is very much a specific KIND of AU, wherein you import traits or characters into a different setting.
Crossovers I see as different from fusion, though still a subset of AU, as they keep settings, traits, and characters as close to their own canons as possible, while allowing for interaction between them. These can vary from letting the Barney Miller precinct have a case that involves the Taxi setting and crew, to actually having a spatial rift that introduces the Enterprise to the Death Star (Enterprise wins, IMHO).
I am far more likely to write fusions than true crossovers. And fusions are one of my favorite versions of AUs, with both canon-divergent and time travel AUs as the other favorites.
(Many of my introspective pieces are canon-adjacent, as I am just trying to get the character mind sets in a 'this could have happened'.)
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Fusion is very much a specific KIND of AU, wherein you import traits or characters into a different setting.
Crossovers I see as different from fusion, though still a subset of AU, as they keep settings, traits, and characters as close to their own canons as possible, while allowing for interaction between them. These can vary from letting the Barney Miller precinct have a case that involves the Taxi setting and crew, to actually having a spatial rift that introduces the Enterprise to the Death Star (Enterprise wins, IMHO).
I am far more likely to write fusions than true crossovers. And fusions are one of my favorite versions of AUs, with both canon-divergent and time travel AUs as the other favorites.
(Many of my introspective pieces are canon-adjacent, as I am just trying to get the character mind sets in a 'this could have happened'.)