A New Faction could be added but to flesh them out would require a great deal more and radically change the Game. I am thinking of simply adding a Grey Alien Unit armed with a Laser to the Zombies that would be Auto-produced toward the end game. Possibly add more than one Alien Unit. There are a variety of different Units in the Game and an Alien for the Zombies could add yet another addition for fun. The Zombies can use more power toward the end game. Like all other living things on the Island, Aliens are not immune to the Zombie Virus and can be infected... why Not?