My suggestion is to have a time of day where harder zombies would spawn. Eg As the sun is setting and/or rising. The zombies should have heightened senses and experience a higher blood lust. They could also form large packs and maybe harder to kill. This time of day would be for experienced or prepared players only. Should only run for about half an hour to an hour and be at the same time every day so inexperienced or unequipped players will be aware and not be on the server. Zombies should spawn around survivors where ever they are, not just inside towns or cities. Also I think the zombies would have slightly better drops eg hard to find ammo or handguns, maybe GPS's. I think this would give a better middle game, as at the moment I think people are getting bored and hunting newly spawned survivors.
