IMO all apocalypse games should have zombies. Why? Because it would make it that much more challenging. Players that play this game to craft and build would usually hide in their village where its safe to do their work, but with zombies they'll have to craft and build while watching their backs against npc zombies. It'll also force them into joining or building a tribe.

Zombies should be spread all over the zones sometimes solo and sometimes in mobs they attack the wild life and any player in their LoS. At night a mob of zombies should be attacking tribes and you must hold them off or they will destroy your buildings and other goods.