Perhaps increase the settler increase penalty for destroyed land. Nations in the 2015 bookmark can quite easily ignore the -60.

I don't think it's neccessary, these days people would repopulate empty cities like crazy :P

yeah, the rapid recolonization of the world would happen in a heartbeat

control zombie horde/ pick were it starts would be cool

is it possible to increase the strength of the zombies?

In modern times zombies at max difficulty with like 1000+ units get whiped out quick. Maybe in the mod add a customization file so people can personally adjust the difficulty to their liking. Also, zombies should pop up in random places, even after the initial outbreak that way it simulates an actual plague, and it makes it more likely to wipe people out.