Zombie spawns issue and variety

[DarthSidious.exe]

So we got a bit of a imbalance here, OZ has next to no zombies spawning due to its large area, meaning the zombies except for the fast ones always stay in the same spot and dont present a threat at all and the sewers seem to have the same zombie pop but due to its nature it mean that your quickly overrun by fast headcrab and die.
anything that can be done for that ? (also, more zombines)