I had this idea that the next fallout would be in some sort of city that has been overgrown. Some big city with skyscraqes which would've come like this big jungle or something. I think it would be nice contrast to the deserts and stuff. Here are some links that the world might look like:
http://img.weburbanist.com/wp-content/uploads/2008/12/postapocalypse_2b.jpg
http://geekoutnewyork.com/images/2008/apocalypse_historychannel.jpg
http://www.asianoffbeat.com/photos_medium/tokyogenso_27_5290.jpg
http://lh6.ggpht.com/_RgDsWB-A41k/S6f4EVgNEbI/AAAAAAAAnkM/2styegVcW00/s640/library,tree,art,miniture,post,apocalyptic,eden-fe516a316ba4da27c0d9824b88d5ec8b_h.jpg