Probably China. I mean, young (governmentally speaking, I know China is an ancient country but it's current regime has only existed for around 60 or so years.) ambitious, rich, and ethically loose country that has massive untapped resources and clout takes the stage from stagnant, aging superpower that can no longer police it own imperial holdings and interests. Where have we seen this before? Oh, well, U.S. taking center stage over the British Empire in the 1950's. Britain taking the stage from the French in the 1750's. France taking it from Spain. Spain from Portugal. You get the idea. It's obviously not as clear cut as I'm making it out to be, but yes, some country will take world precedence over the U.S. eventually. Maybe not in my lifetime, but definitely in my hypothetical children's. Right now, China is a solid bet with Brazil a close second and India holding steady at number 3.