Being a zombie. Imagine an open world sandbox game, where you are the start of the zombie infection. You bite a few people on the streets, they bite others, you spread from place to place and get quarantined... so you have to mass your zombie hordes to break through the blockades to infect more of the world. You could put an RPG element in there somewhere too... "You gain 100xp for biting three people! You are now a level six zombie!" Upgrade running speed, infection rate, strength, jumping height, etc. Then you can have upgrade for your hordes, "Spend 50xp to turn this zombie into a hunter!" Hunter I only used because most people would get the general idea if they played L4D. I know you can be a zombie in L4D, though infecting a large mass of people is something entirely different.