Inspired by another thread. I just... didn't want to touch it. At all. So yeah. Co-op mode. Zombie npc's spawn and try to make their way to the player's tent(s). I think the biggest challenge in making the mode would be making sure players don't break the AI by blocking spawn points, whatever they may be. Stone should probably be scarce to prevent excessive campping. Zombies and their spawners could drop gold that can be used to buy supplies at the tent or make some more advanced defensive implements. A day/night cycle could be used to give the player's a breather. Stuff like shovel-zombies (effective at tunneling through dirt) or an infested airship could be used to mix things up. Etc etc. Of course, a decent enough scripting/modding support and the community can do this!