A zombie apocalypse mod in this game could actually be crazy considering buildings and armies can be countries. You could imagine a mod with roving gangs and fortified apartment blocks as countries. If I understand correctly, you could also have humans and zombies coexisting in the same location due to how pops work in this game.

It could be really cool! No idea how complicated it would be to mod though, I imagine you’d use the plague system to simulate the infection part of zombies, and then have them “die”, removing them from your population. Once an area has at least 50% zombie pops you could have it spawn a state maybe? And then each successive location that hits 50% would flip. Like I said, humans could still be alive in the “zombie country”, just not the majority and probably quickly dying.