This is a idea for the main character of a first person shooter. The game play has hints of horror and gore.
Some sort of scientific experiment or virus has infected you with a rage. You have the ability to totally flip out and go nuts. You have a ‘rage level’ probably represented by a bar on the side of the screen. You rage level is always increasing and to decrease it you have to go nuts (by pressing a button). You go mad and run and punch and jump at people, bite them, pull their heads off, pick up objects and throw them etc. Pressing rage against a wall causes you to run into / through it. If you don’t use your rage and it gets to the top, your head hurts and you clutch it, weird things start happening around you, start seeing things, screaming, delusional. You can also decrease your rage by taking tranquilizers / injections. When you are in rage you don’t get hurt as much and are much stronger. Sometimes you will not want to rage, so you have to take drugs to stop it (don’t want to be found / sneaking around).
A lot of games have reasons why you can’t get killed as easy, which makes a game more believable (or at least the unbelievable things are more believable) and means that you don’t die as much. In Halo you are a cyborg, in quake and doom you are a Marine and in Half-life you have a HEV suit. This game you have the rage.
The story of the game needs to explain your rage, and it probably needs to be a large part of the storyline. I was thinking about coupling this idea with a zombie virus (zombies are always cool). For some reason (needs to be explained) the virus gives you the rage instead of killing you. This also explains why you can’t catch the virus.
Also to turn this into a bit of a psychological thriller you might, not wanting to but the rage makes you, kill your best friend towards the start of the game. It would give you a reason to want to find out how you became this way, and punish who did it to you. And the same person / organisation is responsible for the zombies. Revenge.