Please forgive the double post (even though some time has passed, but I want to place down the notes I have so far on the design of the game)
I can think of several different locations and stories that this game will take place in. But mechanically, it's basically a top-down variant of Time Crisis. (Or any other Arcade Rail Shooter)
The Hero goes through a series of Rooms with bad guys shooting at him, which he must go through within a time limit.
In each room, the hero can only move in a limited space, which the bad guys can't usually intrude on (save for an occasional melee bad guy).
In the space the hero can move around in, there is at least one hiding spot, usually behind a counter or wall, where he can't be hit. He must leave this hiding spot in order to shoot. He must return to the hiding spot to reload. (The Bad Guys can duck behind walls and boxes as well in between shooting.)
When all bad guys are taken care of, the hero automatically moves to the next room. Along the way, the player can shoot at various objects in the room to gain power-ups, heal hit points, get bonus time, and other things before the hero arrives at the next room and--ACTION!!--the fighting resumes.
That's the basics of the gameplay, and I have several different ways to put a story around it.