Prostitute!Harry - Harry spends his evenings standing under a lamppost, smoking cigarettes, and waiting for the next paying customer. Severus Snape spends his evenings across the street, watching Harry.
Kicked out of the hereafter - all Hallows Eve and the veil is thin. Harry is drawn from a celebration at Godric's Hollow into the graveyard, where he encounters (canon character(s) of your choice). Severus appears at Harry's feet, not quite conscious. Character(s) ask Harry to watch over Severus and to find happiness in his life.
False paternity claim - a witch is claiming that Harry is the father of her baby and the paternity test bears is out. He's never had sex with her. Can Snape help prove that she didn't get Harry's sperm the usual way.