I think it would be quite interesting for the gameplay, hordes of zombies that scour the map looking for the player or his base. With this the player would have a reason for traps like spikes or barbed wires. These zombies could leave the cities on a full moon night for example, or simply wander off from any city on any given day. In random quantities ranging from 30 to 50 zombies (on average 40).
Another interesting thing would be zombies that roam randomly on the map, without forming hordes.
I find it strange that a country or state is infested with zombies, but we can only find them in cities, farms or military bases. It would be interesting to see zombies walking the highways or forests with no direction, and when they found the player or some wild animal, they attacked him.