Interesting topic. Few points, some of them already brought up individually in other posts:
(1) it would be more correct to speak of a hydrocarbon era, and when you factor in natural gas... this era will last longer I think than you predict
(2) population - Japan, large parts of Europe and Russia are dying out; China, the rest of Europe, and to a lesser extent NA are "greying"
(3) South Africa - is only ahead of the other African states because of what was built up before the ANC took over; it's much more likely to turn into Zimbabwe than grow into any sort of super-power
(4) Islamists are a non-factor, except as a destroyer of other societies; "Islam" remains stuck in the 14th century - it will need a Renaissance of sorts to become a "power"
(5) Is anyone else struck by parallels between US and the Roman Empire?
(6) Of all the potential "superpowers" only 2 seem to want to be "super" - Russia and China. I discount Russia because of demographics and incredibly inefficient government; China has its own governance issues, but...
(7) Of all the potentials, Brazil is by far the best placed geopolitically - it has no regional rivals, and is only limited by its own wherewithal
Just some random thoughts...
