I love the idea of being able to launch different modules to other planets, which can then be assembled to make moonbases and so on. Perhaps even then lift them onto rover modules, so you can move your base around the planet for whatever reason. In terms of VABs on other planets, I'm not sure about going that far, but potentially being able to reattach parts to rockets once they are landed on a special landing pad or something would be great, so you can launch a rocket from Kerbin to the Duna base, then launch a service rocket from Kerbin to the Duna base with the parts you want to add to the first rocket. Of course, KSP is still only very early in development so I'm not suggesting this be added in .18 or .19 or anything, I'm just saying I'd like to see KSP go in this direction after all the core elements are finished.