The next zombies game.
I was playing my fav game: Call of Duty 5 Nazi Zombies as I have been for nearly a year (its so addicting when the rules are : survive) when I began to think of its opportunities as a full on game. The is so much that has yet to be explored. Add in the ability to turn anything to be a weapon and or manipulated (lockable/seal-able hallways and doors), knock over furniture and convert it to barricades, possibly add in food requirements in the single player / even better than that online multi player campaigns. that imo would be an intense full game.