I only thing China, Russia, or the EU (if it officially merged into a whole country) are the only possible replacements to the US, and it would take decades for this to happen.
I think Isreal is becoming the next world power both in armament, economy and agriculture. The whole world should watch out for Isreal in the next 5 yrs and see what will happen. I am convinced.
Israel is quite a small country to be a world power. They are rising as a world financial & tech hub, though.