Most zombie media has a pretty set formula. A group of people are accosted by the undead, or at least reports of them. They try to find shelter whether it be a house (night of the living dead), mall (dawn of the dead), research facility (day of the dead), funeral home (Return of the living dead)... and so on. However, not all is well in this little holiday from the rest of the world as there is internal division, limited supplies, or someone infected and ready to turn complicating maters.
I tried to make the Anarchy Zones a bit different from all of this. My theory was to try and create a world that lent itself more towards exploration than entrenchment. Varying reanimates and intelligent enemies in the form of aliens prevent it from becoming just a shooting gallery or too safe once the walls were built. Amazing bits of technology like partly-operational nano-factories or the chance of finding working internet nodes is another draw out of the bunker.
The supposedly safe areas are possibly the most dangerous. NEST arcos are in urban areas (surrounded by the dead), are probably partially overrun themselves, with a few fire doors and security isolation mechanisms separating the functioning areas from the others. New Birmingham is a repressive theocracy with a long term plan of conquest. Las Vegas might be safe from aliens and have effectively no reanimates remaining, but there is a notable lack of food, water, and material. he only thing in abundance is military units ready to stat a civil war and generals who are contemplating the use of nuclear weapons to eliminate what little remains of the other nations.
Resident Evil and The Postman may have been major sources of inspiration, but ideally the game should have definite shades of "The Adventures of Huckleberry Finn" as well. Stopping leads to strife, and movement is one of the keys to happiness.