I've felt strongly that there should be more variety in zombie appearance, a bit of a shot to immersion to have a horde of octuplet bikers sauntering your way in perfect unison.
In an ideal world, there would be variance in clothing, hair styles/color, height, even body/facial structures. (basically, as if each zombie walked out of a random character creator session). I'm sure that would be as expensive as hell, and would result in fewer zombies to balance performance, which of course no one wants.
Love the voxel world and the freedom it gives, but gosh darn it the price is high sometimes