I feel like safe houses can survive too easily and that it makes it too simple being able to clear out sections, so I think they should add large hordes which come from off the map and pass through the town, leaving some behind, so you have to survive the wave of hundreds of zombies and at the end there are more zombies in sections you'd previously cleared. (I'm thinking of it like the herds from walking dead, that sort of size, power and idea.)