If Japan does half as good as in OTL (you know, after firebombing, nuking, complete destruction of anything valuable, occupation, ban of research for a decade and such things) it should be superpower number 3. For a good portion of the Cold War Japan had a bigger economy than the Soviet Union (after all those unpleasant things), now they wouldnt have restrictions on their military and constitution - they're sure as hell going to have a big, modern military and nukes by 1955 at the latest.
Of course the word "superpower" might lose any meaning - the USA wouldnt have much presence in Asia (South Korea, Taiwan, Japan go poof), the Soviets are stuck between East Asia and Europe and Japan is holed up in East Asia. I'd think that this would be more of a "great power" world where the European ones simply drop out that category due to the wars destruction and dependence on the USA.
Of course the word "superpower" might lose any meaning - the USA wouldnt have much presence in Asia (South Korea, Taiwan, Japan go poof), the Soviets are stuck between East Asia and Europe and Japan is holed up in East Asia. I'd think that this would be more of a "great power" world where the European ones simply drop out that category due to the wars destruction and dependence on the USA.