1. Post-DH, not-dead Severus keeps waking up in Harry's bed with no memory of going there, and Harry keeps waking up, finding him there, and demanding to know what the hell he's doing there before kicking him out. They're both pretty sure they're having sex, but both insist they aren't doing it on purpose. Options here: sleepwalking/fucking; being under the control of another party; someone is lying.
2. "Look, Snape, I'm sorry you don't want what I'm offering, but really, isn't it better than dying?"
3. Harry, age 25 or so, finds himself actually writing fan mail to a newish author of science-fictiony romance novels. Really steamy ones. The author never writes back, which, not that Harry wants to bank on his name, but seriously, no one in the wizarding world doesn't write back to him, and it's a wizarding publisher. So, Harry goes looking for her. Who knew Snape would take up a second career writing smut?
4. The only potion that will keep Snape alive after Nagini involves semen, willingly given. Once the news of this gets out, the sperm bank sees a sharp decline in donors because most men find this prospective use of it icky. Harry, being Harry, rolls his eyes and volunteers because it's really creating a problem. However, he isn't quite sure what to do when it turns out to be even more effective really fresh.
5. When Andromeda becomes unable to care for Teddy in late 2003, Harry finds himself with an energetic 5-year-old on his hands. He needs a partner for this job.
6. This is rarely an issue, because not that many people share compatible patronus creatures, but it turns out if your patroni mate, it can lead to some surprising consequences. Harry didn't even know pregnancy was possible for wizards.
7. One of the things Harry does in MLE is track down unexpected uses of British-style magic in third-world environments, because it would be so easy for an unscrupulous wizard to take advantage. He's more than a little surprised to find Snape doing school-building work in (eastern Africa, southeast Asia, South America, other locale where western agencies do a lot of this sort of work)
8. AU: Severus Snape, director of university theater; Harry Potter, naturally gifted actor with a problem with taking direction and no discipline whatsoever. Sparks fly.
9. CSI: Hogwarts. Snape is the team leader and Harry is one of the lab techs. Everyone pretends not to know about their off-hours interaction.
10. Severus is a Cardassian. Harry is a Bajoran. Their love is forbidden.
11. After a long and painful healing process from the bite, following on a completely frustrating personal life that's been there forever, Severus becomes a complete recluse, and while it's partly grumpy-old-hermit style, it's also fear--that he can't quite imagine how his life would be anything but a continuation of dreadfulosity. Harry's a little puzzled about his own motivations, but feels the need to help Severus get out into the world again.
12. After a very public, very embarrassing meltdown, Harry is no longer a wizarding superstar. He's more a wizarding pariah. Enter Severus Snape, who offers to help him rehabilitate his image. Harry finds this hilarious, but then figures, hey, it's not like it can get a lot worse.