Working title: "Play"
One-line pitch: An FPS based on kids playing with action figures.
In basic play style, this would be mostly a standard FPS - run through corridors blasting monsters, collecting power-ups, opening doors, etc. Visually, the design would be based around the concept that you are an action figure in a kid's game, and, as kids will do, he/they are making use of everything in their collection of toys.
So the environments may look like any kind of toy at all - a Star Wars-like playset, or playsets from any other toy genre for that matter (military, sci-fi, fantasy, etc.), or build from Lego bricks, or even made out of pieces of cardboard masking-taped together into rooms and corridors. Some levels may just take place on the floor and furniture of a kid's room.
Enemies would also be toys drawn from any style. Any moment you may find yourself facing bad-guy action figures, or rubber spiders, or perhaps the occasional Barbie doll (gigantic, in proportion to a standard action figure) or stuffed animal. Or even those rubber monster finger-puppets (complete with a visible finger emerging from the bottom and disappearing into the floor). Most of them should probably move in that peculiar bouncing puppet-gait children playing with toys always use for "walking".
Other than the visual style, the concept allows for some other unique points. One "power-up" possibility would be for the player to be lifted up out of the level, getting a "birds-eye" view of the playing field. Certain enemies might have a special ability, when killed, to come back to life immediately by yelling "Nuh-uh, you missed!" Somewhere in there should probably be a "Christmas" level, in which a bunch of completely new enemies appear, sequentially, accompanied by obstacles of bows and wadded wrapping paper. I have no doubt other such ideas will emerge during development.
If I had the necessary modeling/animation skills, I'd do this myself as a mod for one of the existing FPS's.