I actually have a notebook with some ideas I saved for a cold day. Most of the time when the idea is rejected isn't because I found it bad, but it's because it's about some obscure game event few people know or care about.

It's easy to fall pray of the easy jokes, Halo teabagging, TF2 spies, Portal cake... when possible, I try to avoid it because either someone made it already, or people are tired to death of it.. Sometimes it pays to dig a bit deeper.