My AU fics are all either canon divergence or apocalyptic/post-apocalyptic - sometimes even both (as in the case of one series I started for Alphas). Mostly I notice specific places where they had things turn out really well by coincidence, and it feels too perfect or neatly tied up, or I think someone made a stupid decision and I want to see what would've happened had they done the logical thing, or I'm just simply curious about the path not taken. In one case (a Rookie Blue WIP that's on hold), I think the scenes as depicted would involve the character in question dying from their wounds, but they have them being still alive when found, surviving surgery, etc., and I just don't think that's plausible - so my WIP is fixing the implausibility.