I got an idea for a new kind of city, quarantined city, just like the Raccoon City in Resident Evil.
These cities are where the zombie virus firstly outbreak. The government tried to quarantine those cities to stop the cataclysm and obviously they failed.
The cities are totally surrounded by very high and thick walls, and there are only a few checkpoints for getting in or out. These checkpoints are usually guarded by soldiers (already became zombies) and turrets. The item spawn rate is higher inside the cities, because no survivor has looted these city since they can’t get in. But the zombie spawn rate is also higher since no one has ever got out.
To implement this idea, I think a few new kinds of map sites should be added: the walls and checkpoints.
The walls take a whole block and there might be some secret passages inside the walls. Similar to roads the walls have 8 different directions.
The checkpoints stands where roads and walls cross. There is a main gate controls by a console, so that your vehicle can get through.
My English is not very good, so sorry for possible grammar mistakes 