Read this:
http://www.cia.gov/nic/NIC_globaltrend2020_s2.html
Countries such as China, India and Brazil are predcited to rival the US if not succeed it by 2050. The EU is also an emerging superpower, infact, if considered a single unit it is a superpower, the EU has a population of 456 million, compared with America's 292 million. The EU GDP is $12.3 trillion ($11.6 trillion for the United States), and it has 1.9 million military personnel (compared with America's 1.4 million). All it needs is to further intergrate and its there. In regards to other future superpowers, I dont think people realise their potential, and in 50 years the US will NOT be the only superpower.