We need more zombie diversity.
Fat zombies would be slow but very resistant to damage (mostly blunt damage) and would be very hard to push.
Strong zombies would have normal speed, vision etc... but are very strong, so they hold the player much better than common zombies, making hard to escape, and deal more damage to the player and structures.
Recently infected would be very fast, have good vision, hearing and smell, with time they become common zombie and then weak zombie.
weak zombie are, well weak, they have the lowest stats and are very easy to kill (this already have in the game, but the player cant manage to know who is who)
Observation: Zombies could become more rotten with time. recently infected would have a white/red color, common zombies the green color and the weak zombies a yellowish to black color.
now about the zombie evolution: the zombies could become a little smart with time, like faking dead corpses waiting for player to aproach (already have in the game but only during fights), zombies making less noise to make surprise attacks. Attack with more frequency barricated windows and doors. Making giant hordes on streets to stop moving cars or even trying to jump on the car hood. Attacking a weak spot, like attack a window instead a barricated one. This zombie smartness could be desativated by the player on the custom sandbox option.