Ok, here it comes.
Tropico 3 and left 4 dead, with elements of Sim City.
It's a slow management sim where you take command of a group of survivors (maybe up to 1,000 for the huge settlements) managing infections (taking them out back and shooting them) forging contacts with other survivors, managing zombie incursions, building up infrastructure so your settlement doesn't collapse and die, and generally trying to reclaim the planet under ever increasing waves of zombie doom, where the goal isn't to necessarily win, but just survive. Watching your ramshackle shanty towns expand into the burnt down ruins of old world settlements, reclaiming old buildings where appropriate. Make it really zoomable so you can watch your survivors toddle around their day-to-day lives, manage their happiness versus freedom versus security as you justify a panopticon state so that no infection goes unseen. Give it charm - streets lit up with christmas lights as makeshift illumination, stadiums turned into coliseums as your now primitive civilisation pits zombie against packs of dogs for funsies. Fight against slaver bandits trying to make off with your people.
Just think about that, how pretty your little post apocalypse could be.