Picture a tribe of aboriginals surrounding a hut and fire chanting and dancing, in the center are 3 chests and 3 spikes with players heads on top. If you think you can get the loot alone good luck, you'll be speared down in seconds if they spot you. Working together with others is a must to get the loot. These NPC's go out and gather resources just like you, you may be getting some wood when one spots you and starts chasing you down with his spear. Tribes would be scattered around the map, big and small. Hunters and gatherer NPC's would be roaming around the map. Players would have to work together more often then not to survive and get gear. idk just something I've thought about for a bit, seems like it might fit the rust vibe a bit better then zombies.
What might look like: