Ok, I've had this idea for a while.
Why not put you in the shoes of the enemy. I'm talking about controlling eight or nine enemies at a time and trying to stop the good guys for once.
Imagine the scenario. You (a group of 8-9 Zombies) are tasked with the mission to take down the Hero. Like most games, each room will be a level or area and each time you will have a chance to flank the Hero who has all the guns and grenades you'd expect in a game featuring guns and grenades. The Zombies (in this case YOU) will only have pitchforks to attack with.
I also thought it would be a good idea to choose a formation before each wave of attack just like a football game.
The other defining feature would be that the Zombies die a lot quicker then the Hero and he has the ability to use health potions. Some of the Strategy would stem from You (the 8-9 Zombies) stopping the Hero from getting to the Health Potions and other power ups.
The best way for the game to work would be in third person like X-com or Pikmin.
Imagine how refreshing it would be to take down the Hero for a change. Being the bad guy is more interesting and would be a breath of fresh air to gaming.