I think RPF fandoms make the best settings for AUs (just because they're 'real world' and therefore the option for diversification without losing something integral is much, much easier). But I definitely agree that some AUs suit a particular canon better than others.

Serial killer or demon/magic AUs or gangster AUs are amazing

THEY ARE ♥♥♥

(I still haven't watched the last season of BFU but I'm gonna. IT'S ON THE LIST!)