40 Year Old Virgin!Snape - or Virgin!Harry although Harry is also SacrificeVirgin!Harry that Snape must rescue (by deflowering him of course)
Auror Harry must find or convince Snape to help him with a case and get over their old feelings of animosity.
Deaged Harry or Snape and the other must become parental figure until re-aged and they find love along the way (I guess that's also kind of related to the more incestuous..."Snape is Really Harry's Daddy but they Don't Learn that until After they've Started a Relationship trope...)
Happy Manipulation by Dumbledore Snarry (ie forced to live together until the reading of Albus' will...or some such scheme that Albus had planned with a twinkle in his blue eyes because he just knows "those boys belong together")
Master Harry since Snape had always needed to serve a powerful Master (or vice versa - Master Snape because Harry wants to reliquish control)
Snarry as Revenge Against James or Still in Love with Lily...