Replace the city mongroup with static spawns of zombies scattered about.
Make spawns dencer at the center of towns.
Centralize the monster despawn logic in a single method.
Make staticly spawned monsters generate a new spawn at their current location when despawning instead of warping back to their origin.
Make zombie population density fall off exponentially with distance from city centers.
Give zombie groups a major bump.
Add some comments to new option flags.
Make static or dynamic zombie spawning an option.