Snarry Mpreg (with Harry preggers) Established relationship, one of the partners uses the pregnancy to control the other. (i.e. Harry uses his prego state to force Severus' hand to marry/stay with him or Severus keeps Harry prego so no one else can have him)
DH: EWE Harry has a stalker, they leave gifts for Harry, write letters, etc, all of which get darker and more possessive. Harry has no idea who it could be. (obviously it's Snape, but since everyone thinks he's dead no one suspects him)
Dark Lord!Severus - with Harry as his pet, slave, consort, and/or right hand man
Dark Predo Snarry chan
Because accioslash hasn't asked for this, I will. Feral Snarry, where one of the characters acts like an animal (but isn't one, nor is a creature) just someone who's feral. With simple answers, calling the other "mate", etc.