Although it's already online, something like Left 4 Dead expanded into a sort of free roaming MMO could be really cool. Imagine a big city where humans and infected were almost more like rival factions, constantly in competition. Goals could involve getting from one side of the city to the other, but you'd plot your own routes, interact with lots of other players, or maybe your bunch of friends could just be one of many trying to get across the city. It would also make playing as infected a lot more fun, the removal of the linearity of the maps would mean that people could get much more creative with where they decided to pop out of as a hunter.