If you want an interesting read about the unpredictability of predicting future events read Asimov's foundation series.
Amazing series. I'm not even sure why I like it so much, there isn't really even a main character. It's a very strangely set-up series, but it's just phenomenal.
As for the rest, I agree, trying to predict that far in the future is impossible at best...but for the sake of the thread, I thought some sort of hegemony in southeast Asia could play a big part, like the EU, should they come to some sort of agreement.
It's boring to talk about Russia resurging...Brazil could be interesting, and Europe has had it's time. In fairness, Africa, South America or Southeast Asia are due for some power. (Though, if you count the Mongols, they had their time, too.)