HBP era Harry hexes Severus when he flees Hogwarts, and S becomes romantically, sexually obsessed with Potter. Draco tries to dissuade him, but Snape convinces the Malfoys that he can deliver Harry to the Dark Lord. Of course he has other plans.
Harry gets Snape's memories directly in the Shrieking shack, and Hermione heals Severus. when Harry reveals himself at the gates of Hogwarts and declares SS to be Dumbledore's man, Snape and Hermione appear and aid in killing LV. Seen as a hero at last, Severus courts Harry, who responds, much to the astonishment of the wizarding world. SS= MoM? It could happen...
SS and HP thrown back in time to Founder's era and cant' get back to 20th c... help in the creation of Hogwarts, and mediate the conflict that drove Salazar out, possibly preventing the rise of Voldemort
Severus must make an arduous journey; a walkabout, perhaps. something transforming, a vision quest, a trial ala Hercules, you get the idea. Harry insists on helping, even though he is forbidden from doing so.
Depressed, postwar Potter and Snape seek anonymity in remote (supply wilderness location) have survival and sexual adventures, bond.