I'd give both my legs for a game like that. Wouldn't need them anymore assuming it was done well.
The closest thing to that at the moment that I've found is zombie horde counterstrike mod on Zombie House server. They track your infects as a zombie and you used to be able to unlock new zombies with different stats and special abilities but they took most of that functionality away. You can build barricades to an extent but there's only so much you can do with a shotgun and a few vending machines.
Building would be a pretty big part of one of those games I reckon. Barricades feature in just about every zombie film I can think of and in an MMO world you'd be talking about thousands of props to keep track of. Must be a way of doing it, just hope it comes in my lifetime
