One possibility is one in which increased globalisation means a planet wide polity, with the people who control America in the driving seat. (By which I means the executives of the multi-nationals rather than the voters.)

Sooner or later the fact that the overwhelming number of people, and the overwhelming amount of resources, within this polity are not in America, should mean that the centre of gravity is no longer in America. Eventually, when some kind of democratic control of the globalised system gets introduced (probably after considerable struggle), it ends up with America as just a small part of the planet, with no greater control than any other part.

Or alternately in that kind of scenario, America pulls back and operates as a separate planet.

And so on and so forth.