Occasional large herds of zeds (walking dead style) and small out of town groups would be brilliant and as Oktyabr points out could be a really good antidote to camping snipers. Also at night you can blunder through forests without a care in the world (aside from falling), having some out of town zeds would make night time in woods a bit more intense. What do you think about having a trigger to spawn some zombies in if the player remains in one place for too long (say 10-20 minutes during day, longer at night)? Also as an antidote to camping and provides another reason to keep moving and exploring.