besides the classic color shifting from a bygone era for zombie variety...how about using attachment points on zombies to add growths with various effects if they're destroyed, detaching limbs on generation, like wandering around without an arm overall hp the same, just a visual effect. the growths though could say release spores that heal/buff any other zombies around if destroyed and instantly infect the player if they're within range. mix and match ^_^ as different growths could have different shapes, colors and effects while being placed anywhere on the model.