Just came across this article:
http://www.rockpapershotgun.com/2012/05/10/thank-you-for-the-dayz-part-zero/A zombie survival mod for Arma2.
It has free roaming, multiplayer (50 per server) and zombies. You can team up with other players and shoot the zombies, or shoot other players or both. Its a fight for survival, supplies and weapons and it features permadeath (you can respawn on a beach, but loose all your acquired stuff).
I think something like this could be a great addon/mod idea for a certain engine, possibly early on once addons/mods can be made for it, as it does not necessarily require complex game mechanics. Just exploring, getting stuff, and trying to stay alive as long as possible. And zombies of course.
But with such a big world it would need lots and lots more players.