Meaning it's either set in the present, the past, or the future?
But seriously, I like the idea of a truly wacky fps. My version would have absolutely no weapons that did damage. Instead, they would do things like reverse gravity, cause you to exchange places with people, reflect other attacks, shove people backwards, make people attract nearby objects, etc. Your starting weapon would be: nothing but your hands, which you could use to throw things at people. The idea would be to use cleverness and the level itself in order to kill people.
Somewhat OT: did anyone ever play the Wheel of Time game? It's only now, looking back on it, that I realize how innovative and unusual it was as a concept. I think something like it on the Wii, using gesture-recognition rather than 40 tabbed weapons, could be a major success.