I would like to do something not-fantasy, since both our regular campaigns are fantasy. I've been playing a lot of Fallout: New Vegas lately, so I am thinking about pulling something from there and making it into a one-shot. Maybe something dealing with Caesar's Legion or the lunatic scientist-brains-in-jars at Big MT. But I also have this vague idea of superheroes vs zombies: build some superheroes and let them just go crazy rescuing civilians and killing zombies, maybe throw in a zombified super-villain or two... Right now, the actual game system I will use is not important- we have several at our disposal, and I will use whichever fits the scenario best.
But ideas and suggestions are most welcome!