There's an
interesting write up of the world implied by the original D&D rules that might have some interesting ideas you could incorporate. Petty warlords, scattered powerful wizards & clerics perhaps left over from the apocalypse, strange monsters for each kind of environment either created or summoned by magic, etc. If you assume the war might have drawn in things from other worlds, you could have a forest like Mirkwood, a desert like Barsoom, etc. The feel of the campaign might vary a lot with travel.