I'd love a Dexter AU with the Sam/Gabriel/Lucifer ship. I'm thinking Lucifer in the place of the man with the Dark Passenger, and with Michael as his nosy brother who works alongside him at the police department. I want Lucifer to go after Gabriel and then realize that Gabriel isn't doing nasty shit for no reason - he, much like Lucifer himself, is punishing ill deeds.
Cue Lucifer ending up in bed with Gabriel and Sam, or ending up in a more gen relationship if you'd rather.
I think Lucifer&Crowley could work really well in the context, too, with them working alongside each other.