[Kenshin851]

My thoughts:

1. Every hour spawns a ton of zombies on the gmap
2. Everything not bolted to the ground is moveable to create barriers.
3. Inner City to defend (most protected wont die as much)
4. Outer City to scavenge (you're pretty much a sitting duck to zombies)
5. If you don't want strictly PVE you can have people select between Classes/Races
IE:
Good Human (Does things without being bad)
Bad Human (Kills other Human players for supplies or cash for supplies)
Zombie (Incredible Speed and Melee will be hard to make balanced vs guns)
6. Repair Kits that you can buy from the inner city or scavenge in the outer city
7. If you're going to do a achievement/experience thing for better weapons and perks.. then. If zombies achieve destroying the inner city they get exp towards leveling. If you're a human then killing zombies and successfully repairing blockades give exp.

I'm sure I missed a few points but this is a starter.