*Post-war first time: Severus owns/runs a business (not Potions related or a whorehouse kplzthnx) that Harry has need of.
One of Harry's children requires a potion only Severus can brew.
Severus wants his memories back and Harry doesn't want to give them up.
Professor Longbottom sets Harry up with Headmaster Snape as he can see what apparently they can't.
Harry tries to get Severus's attention by making him jealous.
*Established relationship: One thinks the other is having an affair, but he isn't.
Harry's let go as an Auror because of a scandal/botched case at the Ministry. Severus has a very bored Potter underfoot.
Kid!fic with any/all of Harry's canon kids or Teddy. Severus slowly warms to his responsibilities as Papa.
Lucius Malfoy wants Severus back but Harry won't let him go without a fight.
Any fic where Severus and Petunia interact for some reason (could be Harry rescued from the Dursleys or it could be Snape-Potter family Christmas-that's not important, it's the whole "I know you" dynamic).