Yeah, that's a sweet idea Almafeta. Could be kinda like 'Escape from New York,' with destroyed/abandoned buildings, wrecked cars, etc. Could find all kinds of 'goodies' throughout the buildings - from weapons and ammo, to food, computers, etc.
There could be certain locations that help the player, like a clinic of hospital that can heal the player. The downdside to this always being available is that the player would have to travel (and fight) to get there. The place also might want a fee or tade for something.
Other places might be a police station (guns and equipment), grocery store (food), gas station (fuel/oil), etc.
There might also be humans still alive, fighting the enemies, or running scared.
After clearing out much of the surface, the sewers would need to be fought in, and that might lead to the Hellgate.
If enough thought was put into it, it could really have lots of neat twists.
