Voxel world is what makes this game the time sink it is, but we should have max zombie numbers within that envelope.
Actually we need zombies capable of destroying that voxel based world, to make the game come to full capacity.
At the moment this is just not the case, and you would perhaps make it the case with super strong zombies, behemoths, godzillas and the like, but the true potential would be released if one could have hundreds of tiny relatively weak single units which when you do something stupid congregate on you and destroy what you were building.
That paragraph right there - $$$
The game as currently set delivers a lot, but not fully on the above, so either way destructible voxel based world comes first. Capable zombie numbers come second. (capable of destroying the world, flattening towns, bringing down skyscrapers and such).
If we cannot have enough weak zombies to do so, due to engine limitations, I would take a Godzilla as well as a best in class replacement to do the same. Would not be as much fun, but better than what we currently have.