My dumbass prediction:
The US (or possibly China, if they catch up fast enough) will be the last superpower. After that, globalism will eliminate national governments. An international government will have most of the power, with national governments serving only local interests. Why not? We already live in a global economy. Perhaps a worldwide regime could be the next step in globalization. It would eliminate the need for national foreign policies and facilitate peacekeeping efforts.
That being said, I cannot claim that my predictions would have any accuracy whatsoever.
2100 is a long time away, and it is nearly impossible to predict that far into the future. When 2100 comes, I will probably be dead and wrong.