This is a really complicated topic, but there are a few things I wanted to get off my chest.
I don't think the US will be the top superpower come that time because so much of the economy relies on oil manufacture and import. By that time, the oil will be gone. Also, most big American companies tend to be short sighted and inwardly focused, which always spells doom over the long term.
China could potentially fill the spot, if it wasn't for the fact that the government can't actually control business interests through law. Essentially, Chinese economics is less about an entire country's gross domestic product, and more about a few very wealthy businessmen within the country, and almost the entire rest of the country is poor. If the mass Chinese population revolts due to the sheer amount of pollution going into the water and air (like I imagine they will) and goes green totally, than they probably could become the major world power, just based on population size alone.
India has a huge amount of potential, especially due to the size of it's population, but the country's lack of geography is a bit of a hindrance.
Feel free to correct me with facts, but NOT opinions. I learned about this stuff in college, but it may be out of date. I just don't want any Faux and Friends stuff.