China, India and the US will be the dominant powers. Beyond that I think that there are two paths we can go down and they all depend on alternative energy. If, by 2050, the world is no longer dependent on fossil fuels, then life will be better than it is now. If we are still heavily dependent on fossil fuels we will be facing a number of reigional conflicts that threaten to create WWIII (just as WWII was created out of several resource based regional conflicts). Hotspots to watch will be Burma, Venezuela, the Arctic, Nigeria, Northeast Asia, and Central Asia/the Middle East (duh...I think the Middle East is permanantly on any list of "hot spots")
On the side note of space travel, I think that one day we will see multiple space elevators on Earth's surface. Indeed once nanotechnology makes a space elevator truely feasible every superpower will want one.