I was hoping that enough zombies would swarm in from other spawn points that sitting on just one wouldn't offer that much of an advantage. In practice, it seems they aren't good enough at finding their way to the player from across the map. I'd like to encourage the player to stay away from the walls in general, though, so possibly backing into one should scatter bullets more randomly. Currently the character only shoots within 90 degrees either side of where you're aiming while in contact with a wall, which in hindsight still averages out as a useful cone of fire given that the zombies take a while to close in.
Thanks for taking the time to try that out, by the way! I think I got into the habit of sticking mostly in the central area while testing this (since I wanted to confirm the helicopter was flying in as expected) which doesn't appear to be representative of how people are actually playing it.