Well, China and India are often quoted because of their population but I don't think they can be in a near-furture on a worldwide scale.
Imperialism and superpower are not the same thing but they are usually not so far from one another. So we might get some hints from that.
China is developping its imperialism on the "regional" arena (South China sea at the moment, Tibet (albeit Chinese people would not agree this is imperialism) and some much more discreet influence in central Asia and across the Amour river, mostly commercial or logisitics-based (China needs oil now).
On the other hand China's population is not really booming anymore but is even getting older and being wealthier and more urban, it won't probably change into a young booming population anytime soon. That is important because imperialism can often be correlated to a young, developping population (both as a mean and reason). Of course this is not automatic but that would hamper China becoming an imperialist superpower.
India has the population, a strong scientific base (probably better than China) but is less "industrious"
than China. More seriously its main pbs I think are its poverty, its tradition (not really expansionnist politically/militarily speaking outside the sub-continent) and its exhausting war stand in front of Pakistan that focuses too much its attention and scarce resources so I don't think India can become a superpower, except on a dice throw attempt which could be real bad for the planet. On the other hand myabe Indians will tire of doing the job in the Persian gulf for muslims but as long as Arabs get the oil money they are pretty untouchable.
I agree with allhailIndia though that their interests might be more regional than world ones.
But I actually don't see the US loosening their grip so soon. For various reasons :
First I do think the US "golden age" is gone and I think by itself the system should already be down now but it resists for :
- financial power : their dominance in a worldwide economic system means that they can actually take parts of the benefits of growth in other countries/areas, diverting resources to remain the main power. England was the first country to use that sytem and it sure enables one country to remain a power for a longer time. The modern economical and financial system is definitely a way of remaining the superpower (might have been ibn Laden's perception too)
- population : the US does the same with population since it still gets immigrants. The population is young, active and enterprising, the brain drain enables the US to remain at the top and to keep contact with other parts of the world.
- cultural : should I say more, albeit the US are changing, US references still are and will be, even after a possible drop-out, like Roman traditions were still widely recognized for a few centuries afterwards.
- lack of contenders : the USSR collapsed trying to keep up and it would be difficult to find a new one ready before a while, even if China does become the workshop of the world.
- global acceptance and recognition around the world. In spite of the hate US policy generates in some countries most government and people around the world don't have too much of a pb with the US dominance. I mean they prefer that one they know to another one they don't know and that could be really different (ie the EC whose values are closer to US ones than to Nigerian or Indian ones for instance). Of course US dominance is not always subtle and some policies (Bush being a paramount of it here) show both a lack of respect and understanding (or simply knowledge, wasn't Bush jr proud of not having a passport before being elected) of local values and ways of life that is extremely irritating.
So I think the US can remain a superpower for still a while, even though through the contribution of the rest of the world and not automatically representaitve of the helath of the American system.
But it will be costly, financially as well as politically, militarily, etc... And being the only superpower is the "best way" of attracting negative attention and bad reputation.
And that could become real bad. So I would see the possibility of a main superpower supported by a ring of regional (Egypt, Nigeria, Brasil,China ...) or second-rank world powers (UK, France, Japan (still for a while at least). That would divert the financial, reputation burden and would allow for a more global acceptance of international rules that would seem less unilateral and more intercultural (ie Egypt in the muslim world).
The UN could be the place for that by granting permanent seats to new members provided the US give up on their "manifest destiny" that they won't be able to support alone for too long I think. So exchanging control/support for time
Of course these are just some ideas. I am open to constructive criticism, hope I don't sound like one these young chin-up guys
o ) and that this is not mere critics at the US from a frustrated old European.
sorry.. 
