(1) Non-magical AU with Snape as the law enforcement and Harry as the snarky loner.
(2) A number of years after the war, Harry finds Snape in a completely Muggle environment. He comes to realize that the cost of surviving the snake bite from hell was Snape's magic.
(3) Time Travel fic. Snape dies, like JKR wants but through an accident some years later Harry gets thrown back in time. Does he do enough to save Snape from the mistake that is the Dark Lord?
(4) Kink fic. (Come on, ya'll had to know this one was coming!) The harder core the better. Not gonna say a specific kink, because really, for it to be good it has to be author's choice.