Well ultimately you can fit any race into any world if you word it right. I think it should come down to the PCs in your campaign, after a race or class is generally a personification of some ideal the player has. You have someone is sees himself a a super intellectual arch-type will make his race/class drastically different from a brute player. So I'm thinking you should have your players produce at least three words to describe the race they would like to be (assuming they can't make it all up themselves) and provide this for you as a guideline to creation. Now, as for the civilization annihilation and subsequent rebuilding there of, i had an idea for the bad guys. With the world in such a weekend state trying to recover and rebuild, a race of intelligent, opportunistic, vulture-esque parasites, whether literal or figuratively, have invaded from another plane, world, planet, whatever. As the PC's attempt to grow a new civ, these creatures at first infiltrate and attempt to manipulate and grow the civ to a thing that would welcome the new overlord. Later after the PCs uncover and stop such a plot the conflict can become more direct, till of course the climactic battle with said overlord.