Well, lets jump into this.
Zombie mode! Yes you dont have an inventory your hunger and thirst wouldn’t go down. A new non-talkable murderous NPC with random weapons and you eat them.
And you could play as special zombies, hitting a button {D maybe since you don’t drink} using their special abilities. Like Necromancy or Hulk Smash!
Also, random spawns of an Item called: Mutate Goo! An item that brings mutations to the undead, maybe changing your type.