I don't believe there will be another superpower after the supremacy of the United States is eclipsed. As world economies become more and more entwined, the differences between nations begin to fade. The concept of a superpower is primarily backed by military force, and a war in today's world to define the position of superpower would be catastrophic for every nation involved, no matter the victor.
The United State's Navy has the ability to go against every other navy on Earth and defeat them all. Just one carrier air group represents more projected air and sea power than the combined navies of China, Russia, India and Brazil. While an American war of conquest against China would be politically and militarily impossible, Chinese aggression against American interests would be equally futile.
Also, I highly doubt another Greater East Asia Co-Prosperity Sphere is ever going to be possible. There are very real animosities between China, Russia, India and Japan, and none of those nations have the ability to militarily or economically force any of the others into submission. The close proximity of those nations is at odds with the vast breathing room afforded the United States by the Pacific and Atlantic Oceans. Proximity breeds friction, especially between such individually powerful nations.
Today there is no single national economy, and there really hasn't been one for the last century or so. When one powerful nation's economy suffers, it creates a ripple effect. This effect is strongest when the United State's economy suffers, as it is really a center point(but not the defining point), of the world economy. However, even Americans feel the pinch when a trading ally suffers, albeit the effect is not as widely publicized. This rampant and still increasing economic globalization limits the strength of any single nation, and will continue to do so to greater and greater effect.
