From a biological viewpoint, I wanted to suggest having zombies with more muscle memories/vocal memories. This would help break the monotony of their animations, as well as add REALLY creepy dynamics to the game. I.E. A zombie female that keeps crying. A nurse that randomly asks "How are you?", a zombie that is looking for his/her son/daughter and shouts for them, a zombie if they hear you say "Hello?" in the darkness. Muscle memories would include waves to the player, bows to the player, wiping sweat, hands covering face after damage, wiping tears, etc.