As it stands, it looks like 100 years from now the superpowers are going to be China, India and the EU--if the EU holds together. The 20'th century was the US's century. It was the largest unified country with a fully developed economy. Obviously there is going to be a point in time where it's absolute power is eclipsed by significantly larger countries. Regions like rest of Asia, Africa and South America are too fragmented for wield real power. The wildcard is these regions forming federations like the EU.