I did a search for “metropolis” and this thread came up.
I think Cataclysm should have an option of either spawning in a large metropolis type area, essentially concrete jungles that take a lot to escape from, or perhaps it could be an optional side-game like the current “Defense” mode or the Dungeon Sprint mode in Crawl. Essentially, my thinking is a “Dungeon Sprint” like side-game, were you spawn in a city like Manhattan during the initial outbreak, and your goal is to get out of the city alive or possibly wait it out over the course of a few months. Z’s run around, kill NPCs which then turn into Z’s, mayhem everywhere, lots of cars jammed in the streets, messed up subway systems and underground tunnels, tons of shops to loot, high rise buildings with 100s of floors, etc. Maybe you have the optional side-quest of locating your spouse and family before escaping.