Here's a thought:
At the 1924 Washington Naval Treaty...
(a) Japan doesn't have as onerous impositions put upon it
(b) the issue of Qingdao is settled differently, with Japan withdrawing from Qingdao and abandoning its influence over the South Manchurian Railway and Shandong Peninsula in exchange for a cession of the Liaodong Peninsula and Kando region of Manchuria (a traditionally Korean-settled area).
(c) Further on, Japan agrees to withdraw from Fujian in exchange for Zhoushan
(d) China goes Communist/Socialist (sort of) in the early 1930s due to a merged LeftKMT-CCP Government (this is me limiting butterflies and thus there still being a PRC) purging the right-KMT. China's name is officially changed to the People's Republic of China. Shanghai, with its Japanese Garrison, is carved off as the Shanghai Free State following the January 28th incident.
(e) The last holdout of the Right-KMT is on Hainan, and they invite the Japanese onto the island to protect them. The Japanese seize the island due to opportunism and it being an important logistical station for seizing Indochina in the name of the allies/Free France. Many Right-KMT relocate to the island like they did to Taiwan OTL. At first the island is a puppet/client regime, but over time Japanese settlers and business interests become a larger and larger presence on the island and the island is incorporated into Japan.
(f) As decolonization rolls around, Japan buys Guanzhouwan from France rather than France ceding it to the PRC in 1945.
(g) Japan purchases Macau and East Timor from Portugal as well.
(h) The Japanese convince the Dutch to Part with West Timor in exchange for being a buffer between the Netherlands New Guinea and newly-independent Indonesia. The Japanese also prop up the Republic of South Maluku. Plus there will naturally be a trade agreement between the Netherlands and Japan which enables Japan to benefit from Netherlands New Guinea's resources. Down the line, when the lefty Dutch government in the 1970s finds overseas holdings unfashionable (like with Surinam OTL), the Japan swoops in and takes the place off of Amsterdam's hands. Besides, by this point the economy of the area is very tightly bound to Japan.
Also, Japan was somewhat open to Jewish immigration in the 30s and early 40s (The Shanghai Ghetto, Proposals for settlement in Manchuria, Chiune Sugihara, etc). Perhaps Japan takes in a large number of Jewish Refugees TTL. OTL Settlement proposals ranged between 18,000 to 600,000. Sugihara saved 6,000 OTL. 23,000 made their way to Shanghai OTL.
Altogether, Japan proper (that is, excluding Shanghai which is an independent state under Japanese protection) has some 310 million people I'd think (150 in Japan without WW2 deaths, 90 in Korea without WW2, Korean Famine, the Korean War, 20 million in Liadong, 20 million in Taiwan, 10 million in Hainan, plus 20 from all the other areas).
Also, with Japan being part of the allies TTL, I think they'd urge for Ethiopia to get Italian Somaliland. The Japanese liked the Ethiopians and I'm sure Japan would enjoy having good trade relations. They'd likely play a signficant presence in the East African theater. Ethiopia is thus federated between Ethiopia proper, Eritrea, and Ethiopian Somaliland here.
I wasn't sure of what to do with French Indochina. I gave Thailand a boundary further east because France agreed to such a boundary OTL and that was only revised in face of WW2. I'm not sure Japan would be eager to give France her colonies back and Roosevelt OTL tried to give Indochina to Chiang. French Indochina being made into three independent states seems likely to me.
I also gave Zanzibar to Ethiopia. I though an Ethiopian Zanzibar would be cool, and I'd imagine that the Sultan wouldn't mind being part of the Ethiopian Federation once the British abandon him as they did OTL (it certainly beats his OTL fate).
Roosevelt OTL tried to give it to Chiang and the Japanese may not be eager
Yes, I know the map isn't quite accurate. I used a 1960s basemap but this map includes a Japan with mid-1970s boundaries.
