Answer:
Japan and possible china.
Because Much to the dismay of the Chinese Communist Party, Japan is still a very powerful regional player in Asia. Japanese pop culture has pretty much dominated the region for the past 30 years, and shows no sign of slowing down. The Japanese SDF might not be as powerful as the Chinese or American military, but it certainly isn’t something to scoff at, and the Japanese economy is actually doing fairly well, all things being considered. The actual reason for the Japanese economy slowing down, is that it actually did so well, it started doing bad (to keep things short).
So how could it become a superpower? Well, it already has the economy, the soft power, all it would really need is the hard power. Meaning, a large investment into the Japanese military. By investing into the military, Japan could cultivate defense treaties with likeminded Asian countries, looking to defend from whatever idealogical threat they might be facing (likely China). Their current relationship with the USA actually places them in a good position for this. America has no interest in opposing Japanese military expansion unless it maybe conflicts with their own political interests. Meaning, if Japan actually cozies up to the USA, America will likely support them as an alternative to the threat of a rising China.
With a pacific oriented military alliance and the backup of the USA, Japan would then be a serious contender as a world power. After that, it would be a matter of what Japan wanted to do with that power that would one day bring them to superpower status.
