What territories and military resources does this Imperial Japan control? Certainly they have the Home Islands, Formosa, Korea, and several smaller Pacific islands but do they have Manchuria? Nanking? (former) French Indochina?
I tried a TL about a Greater East Asia Co-Prosperity Sphere arising under different circumstances as a similar venue, at its peak in the 70s it spread from Persia to Palau but fractured as India and Indonesia wanted more power. China was controlled by breaking her up into half a dozen states of varying levels of fascism and development. Looking forward to seeing the ideas here!
I would think the home islands, Taiwan, Hainan, Korea, Manchukuo, Chishima (Kurils), Karafuto (south Sakhalin), and Micronesia at a minimum if the Soviets aren't the ones driving them from the mainland.
Eventually the Japanese would have to cut it quits in mainland China by the 1950s I think. There'd be no end in sight to all the warring. I imagine they'd want to keep Shanghai as well.
Over time Mao's communists probably would end up being the ones to unite China I think, considering how they'd likely be the most organized, least corrupt, and most well-funded group.
Japan would probably look to newly independent nations in Southeast Asia as potential trade partners, but I'm not so sure they'd find much luck. Cambodia wanted to be nonaligned, Vietnam has its issues, Laos is landlocked, Malaysia is close to Britain, Brunei is a British Protectorate, Philippines is in the US orbit, and Thailand joined SEATO historically and mainly aligned with Japan because it saw the writing on the wall.
Indonesia might make sense as a friend of Japan if they go for the whole non-aligned thing.
Japan had an interesting dynamic with Ethiopia I believe.