Recycling some of my old prompts as I would love to read some of these ;)
1) Slughorn has abused members of his slug club before (Snape specificially). "You said he'll try to collect me. Do you want me to let him?" Severus discovers that the headmaster is willing to use Harry as bait in order to find that memory and he is appalled at such a breach of trust. How does Snape prevent Slughorn from doing to Harry what he did to him all those years ago? Maybe he polyjuices into Harry and takes his place, or something else? You decide :)
2) "People change in the maze." After returning from the graveyard, Harry is examined by Madam Pomfrey and she discovers he's been raped. Everyone assumes it was Voldie or a DE, but it was in fact Cedric in the maze. Harry doesn't know how to tell anyone what really happened as everyone considers Cedric a big hero etc. Severus discovers the truth during Occlumency lessons.
3) Harry's parents are alive and James is the ambassador or something like that in a foreign country. Harry is bit wild, staying out all night etc. and James gets kidnapping threats. He hires a bodyguard for Harry, the no-nonsense Severus Snape. Harry is bored as he is not allowed out anywhere anymore and decides to seduce Snape as a cure for his boredom. Bonus points if Harry is a virgin but everyone thinks he's been very promiscious.