Kinda liking the idea of doing Kids and/or Twins. The player would run around as one and the other will follow him with AI and attack whatever he attacks.
Still thinkin about their ability. Somethin like ripping boils of themselves and throwing it at the survivors poisoning them or blinding them. And they could have deformed jaws which makes em able to bite and latch onto a survivor. The survivor will still be able to run around but will lose health quickly.