-Inversion of Snape rescues Harry from the Dursleys (Harry helps younger Snape, through some means of time manipulation, your choice.) How does this change the history of the war? (Eventual slash would be nice...)
-A and B strike up a correspondence, one that's anonymous, but only at first. Could be war-time, post-war, or AU.
-Reincarnation of a pair or group of characters wherein they maintain their memories in their new lives. Perhaps some odd artifact from the Dept. of Mysteries is responsible. Must work together to survive in a vastly different future. Will they ever return to their old lives?
-EWE: Something happens to Andromeda, and Harry raises Teddy.
-Harry has always had help -- and it's arguable that he doesn't truly become an adult before the end of the series. What forces him (or helps him) finally grow up?