In all likelihood, I'd expect it to be a country or a group of countries that we would never really have expected.
Unfortunately I don't think the US would go down quietly if the empire really is failing. Another major war wouldn't be out of the question, clearing the board for a lot of change to take place.
Without a war though, and one can only hope this is the case, I don't see China pulling it off. I both don't think they really have that motivation, and I don't expect they really could transition from such a totalitarian state to an empire driving anything near the level of US dominance.
When the US empire fails, and it has to eventually even if we aren't currently walking that path, the question is whether it falls apart or fails violently. You could point to many parallels with the Roman Empire and guess that it will just fall apart, even if there are fringe wars along the way.