a game where you only have a single bullet for each level. And the levels are basically some sort of puzzle So you have the choice of using the bullet to, I don't know, kill an enemy somewhere, or flick a switch?
we could go super sci-fi, and say your gun draws all the iron from the atmosphere to make bullets, and the atmosphere is low in iron?
"You Can Only Have One Brain!" "You Can Only Have One Eye!" "You Can Only Have One Lung!" "You Can Only Have One !" one for each level? :D The lack of each respective organ impair your ability throughout the level; could do it so that you could do it so that they lose one organ per level throughout the game, gradually making the levels more and more difficult?