This is a great question. I am sure that you will get many different answers. In my opinion, I think the next economic super power will be China for three reasons.
First, China is a creditor nation. From a historical point of view, power and economic development follow money. China has lots of it. Remember a few weeks ago, the EU asked for a bailout. I think the next century will be the century of China.
Second, China is effective in their government. To be sure, there are problems and scandals, but at times it is more effective than the United States, where politicians are always aiming to get votes. All of this matters, because China is strategic in getting things done. For example, China is buying natural resources and things that would make their nation great. For example, they are buying land and mines in South America, Australia, New Zealand, and many other places. They already have a stranglehold on rare earth elements. Moreover, they are not making vanity purchases like the Japanese in their economic strength.
Third, they have lots of people. So, there is a domestic market that they can tap into. This will cause further growth.