Let's say the US slips up somehow and loses its hegemony. What are the most likely groups to step in and take advantage of this, and how might the situation manefest itself (would we get another hegemony, a bipolar system with a weakened US as one pole, a bipolar system with two non-US poles, a multipolar system? How might it affect the economic situation in the US, the political climate, etc?)
In the immediate future, I can only see a multipolar system emerging from a reduction in US power. China is powerful, but seemingly disinclined to act much beyond their borders, though that may change with the US no longer providing security in certain regions.
China is the only nation that even has the potential to step in as a successor to the USA as a global hegemon. But their military are not up to the task, and the (lack of) development (and pollution) of rural areas of the country is also capping their future potential.
Russia is still a power to be reckoned with, at least in the Near Abroad. It's army is also not up to full potential, though scheduled weapons systems purchases should act as force multipliers. Throw in their nuclear weapons, expertise in large-scale hacking, energy resources and economic growth, in addition to their cultural influence in the Baltic/Ukraine/Belarus/Kazakhstan and military basing in Central Asia and Russia will continue to be an influential global player.
India is on the rise as well, though much slower than China, and is mostly being supported by the USA, as a hedge against any Chinese militarism in the future. Still, India has even more problems than China, and less advantages or resources to deal with them. India is also constrained by nuclear China on one side and nuclear Pakistan on the other.
If the EU gets through the current financial crisis, it will be in a much more unified form. However, that it will get through the crisis is not a given.
Brazil is lagging behind India, but also on the rise. Nevertheless, poverty and corruption are going to be a problem which will take a lot of work to solve, and will be a drag on their overall economic growth, as will their education system. The oil deposits off the coast are no doubt a welcome find for the country. There is also an element of cultural isolation, Brazil will not have a lot of soft power options in South America, other than economic ones, given their varying colonial pasts and linguistic differences.
Other players that may form "regional" poles of power linked to the above great powers would include Turkey, Iran, Nigeria, South Africa, Uzbekistan, Japan and Vietnam. They dont have global reach, but their actions could have global repurcussions, for good or ill.
I can see the US seriously losing its shit once it realizes it is one great power among many. "American Exceptionalism" is the unofficial ideology of the left and the right, whether it's "liberal interventionist" or "neocon" in flavour. The US will likely not accept constraints on its power and ability to act until they are made obvious, ie; via military defeat. I can see any period following such a defeat as one of economic instability within the US, political unrest and likely an increase in terrorism and political assassinations. Think early Weimar Germany, and you're not far off.
