I think its going to come down to something very much like
Land of the Dead. Fortress-like cities held by whomever is powerful enough to defend them. One might be held by a cabal of powerful necromancers, another by paladins and clerics. One might be hedonistic carnival like Masque of the Red Death, another a grim, taciturn military socitey. The people in each place would essentially be held captive, whether their benefactors are kind or cruel, there is no alternative but the wasteland of the living dead.
And even inside the fortresses, there would have to be precautions. Anyone dying is a potential outbreak. Teleportation level magicks would be the only real way to travel between the fortresses. Trade would be limited, communication sporadic.
There would also be a lot of the world not protected by those fortresses, lots of cities abandoned by all but the living dead, and small pockets of survivors hiding out in the wilderness. So there'd be lots of rescue missions, lots of looting missions. The pc's could be sent out into the wastes to recover lost things and search for survivors, and as they go up in levels they might form their own fortress-city or take one over (either as a reward for faithful service or by force).
I imagine with the small number of survivors (I'd think even a large settlement would have less than a few thousand people) and the small amount of usuable land, most people are going to have to rely on magical means to produce food. Low level cleric spells, and items like Endless Water Decanters, Murlyun's Spoons, as well as Plant Growth spells will all get a major workout.