Am I the first? Whoo-Hoo! These are recycled from another fest: 1. Though the rest of his colleagues think he should stay at Hogwarts, Severus explores other employment opportunities after the war, with varying levels of success.
2. Severus catches a glimpse of (insert your desired canon character here)’s bare feet and he has become obsessed with them: he worships them and wants to do all sorts of naughty things with them (and eventual the rest of their body).
3. Severus meets a former Hogwarts or Order colleague or student (insert your desired canon character here) somewhere outside of wizarding Britain in a club (it could be a dance, book, scrapbooking, or BDSM club--let your imagination fly!). They decide to pursue their desires with no obligations, no strings. Of course, they both fall for each other.