Game idea: Arena shooter, with RPG elements. Sounds like its been done to death but I propose to have an array of different weapons, armours and equipment. Things like teleporters, grappling hooks, grenades, mines, invisibility, shields etc etc.
Also crucial is the notion of gradual decay, so equipment/armour will become damaged, may a gun might jam or even back fire ! Get hit in the legs your movement will be reduced, get shot in the arms your aiming will be less precise. Hopefully this will lead to a tense game of cat and mouse, with an array of different load outs and tactics emerging.
I am not convinced that anything like this exists out there, maybe I am wrong, what do people think ?