The usa like each super power in the world is going to disappear according to the principle of circle of life ,and follow Rome, The Islamic kingdom, great britan etc..
Well Russia has always been a close second militarily. If we're talking about economics then Germany will always be number one. They are too frugal and self sufficient to ever start a war again though.
China will be the next superpower reasons because China has the second largest economy after the US. Also China is building up its military to be a capable force, however still years away in technology.