By poaching from everywhere and using it's original sin to catapult it into the stratosphere. America was built to supposedly combat Imperialism and classism but started off by subjugating human beings and committing mass genocide. Then went and fought the likes of Nazis only to give some refuge in return for their cranial assets. The whites who built America wanted to limit the power of big government to escape being permanently underclassed by the likes of ruling families in Europe. This idea is what the greatness of America is built on despite the obvious hypocrisy of subjugating others in the process. For all it's faults as a society America has still championed the idea of equality in a way that other Big Nations in the world do not.This country has been #1 in (nearly) all aspects of civilization for almost a century. If it were to turn into one of several failed Nations around the globe, which other Nation would be the objective best to take it's place and why?
To answer your question the nation that would take it's place would be China. Ushering in an era of totalitarianism by mere influence. whoever leads the world as a nation ends up having a pervasive influence in other societies down to politics. For as much as I dislike parts of America I see no better leader in a world dominated by stupid human beings. Simply because America still values freedoms in a way many of the other leading nations don't.