Only China has the GDP to replace USA. So while the answer isn't written in stone, it looks like it'll be China, or since China favors UN governance, maybe we'll move to an old discarded idea (because it didn't serve American interests) like an international currency system that Keynes favored.
https://en.wikipedia.org/wiki/Bancor
Recall that America is currently attacking Iran without provocation and is aiding a genocide.