DayZ (hardcore, lawless, survival), but without the cheating epidemic that ruins DayZ, and with more tools to allow for attempted civilized interaction with other survivors.
what would you like to see in a zombie simulator?
Also, infection. You get bit and you've got to hide it from your crew, lest they shoot you before your time is up.
Massive player counts could be cool if the world was also equally massive.
The zombie setting lives from the doomed, forlorn setting where only a handful of people survived, there's no place for massive
DayZ avoids crowding by 'sharding' 100,000 players into few thousand world-instances of 50 players each, so that on it's 225km^2 map, you've got a good chance of being alone. This works -- meeting another player is a rare event, and personally, I usually hide or run because of the 'stranger danger'. Can't trust anyone if they know you've got beans!
If instead, the world was 10000 times bigger and no 'sharding' was used, it could be pretty damn epic. It's common in the apocalypse setting for people to be hearing news of places 100's of miles away (via rumour, or radio, etc), which results in a "quest" to reach the source of that news, which takes a very long time, seeing that working motorized transport is now rare. This doesn't work in DayZ, because I can walk anywhere in the world in about an hour or two.