I've had an idea for a while, a post-apocalyptic shooter starting in city 17 after the citadel goes boom-boom. You play a rebel that is trying to get out of the city. You would be going through the (mostly) deserted streets, subway systems and rooftops. You would not be a one man army like Gordon Freeman, and you would be switching weapons like in Left 4 Dead. Enemies would include metro cops, zombies, zombine, and possibly some other humans.