I think it'd be cool if the safe zones weren't always safe. Like they could be raided because that would be realistic. Humans gathering in one spot? That's a perfect enticing buffet for zombies. Maybe put the raids on a timer and the quicker you do something the more you get rewards for it. Like say if you keep them way in less than 5 mins you an now trade for some super awesome gun you can't get it if you don't have something to trade but you COULD if you did. That'd be neat I think. That way zombies could take over more of the map for the whole apocalypse feel,