The BodySwapping Idea
Set some time after New Directions wins Sectionals, and Kurt still has a crush on Finn. Finn admits to Kurt that he wants to be someone else right now, after the whole Quinn fiasco, Kurt sympathizes with him, and the next day they wake to find that they've switched bodies. Kurt-as-Finn has to suddenly deal with running a football team, being pushed to the front of Glee numbers and Rachael, while Finn-as-Kurt finds himself alienated as the school's resident gay guy. Of course, having to spend so much time with each other leads to certain feelings coming out.
I came up with this just because I love bodyswap fanfics, and I like the idea of writing a romantically-driven one as opposed to the simply-humorous ones I usually write.
The MPreg Idea
A twist on Kurt's statement "Well, at least I'll never get anyone knocked up."
Finn and Kurt get drunk and end up sleeping together. The morning after, feeling extremely embarrassed, they decide never to mention the 'incident' again. But things don't go so smoothly, resulting in Finn becoming pregnant with Kurt as the father. Not exactly going to go down well at McKinley High, is it?
This idea was heavily inspired by a Jared/Jensen mpreg fanfic I read, http://memphis86.livejournal.com/125317.h