I really think the term "Superpower" needs to be redifined. Personally I think it can really only be used when there are maybe 1 or 2, possibly at the max 3 candidates. Any more than 3 candidates than you no longer have a "Superpower", instead you have more of "Globalpowers". The difference being:
1) Superpower - Nation that can exert it's policies and military influence on the rest of the world with little opposition.
2) Globalpower - Nation that has the potential to exert it's policies and military influence but is kept in check by other globalpowers.
See during the Cold War, there were Super Powers because only the U.S. and the Soviet Union had tremendous amounts of power, no one really kept them in check. However now, a lot of nations are getting to the superpower status and they all continuously can keep each other in check, thus making them more "Globalpowers".
Anyway these are my candidates for Globalpowers in the next 50 years.
United States- Large country geographically with a democratic form of government. Capitalism helps the economy and they have a very strong military. Good relations with most European countries and good trade partners with Asian countries like China, India and Japan.
China- Large country geographically, booming economy due to large workforce and large number of exports. Advanced and large military.
India- Also a large country geographically, strong economy that is set to expand even more (by 2050 is predicted to have the 2nd GDP in the world, passing the United States). Also has a large workforce of engineers, doctors, IT professionals. Has a strong standing military and is a democracy.
European Union- Not a country, but a group of countries that together produce a large chunk of the world's economy. United has a strong military force and is a diplomatic union.