Actually what I would like to see is a more graduated spawning system. Have an area marked around a town or city with a special spawn process. one based on the number of other zombies in the area. Kind of like a growth pattern, the more zombies in an area the faster they will respawn to fill the area back up. If you go into a town and only kill a few zombies, say 10%, then it will fill back up quickly but if you killed 90% of the zombies then it would take several days to completely respawn.
Of course, as I was thinking it through there might be a simple way of doing it, if we can convince the Pimps to add a variable for Max Number of Spawns in a Day. Set the MaxAlive to 50 and the DailyRespawn to 3 and you would have a case where you can 'clean out' a town of zombies for a few days but if you leave it for a couple weeks, when you come back it is full of zombies again.