There's nothing that I can say that others haven't already, like the tech tree and whatnot. Maybe flesh out the ruins a bit more or have some that you can explore, like dungeons in FFH2. The setting for this makes it unique, so you can basically rob whatever features you want from other mods and it'll still be very original. By that Planetfall comment, I meant that this is how Planetfall started out. Bits and pieces until it basically became one of the big Civ4 mods. A post-apocalyptic mod for Civ4 is a great idea and to build on it you'd just have to use what's already there.