My best idea on them is to change Japan in the 19th Century, and not treat the Koreans or Formosans as serfs, but instead try to raise the idea of them being the brothers, born from the same Gods (which in the Koreans is actually true - the Japanese are genetically descended from the Koreans) which Japan is seeking to unify. So, while Japan still comes down hard on troublemakers, they do not look down on the Koreans or Formosans. This leads to Japan having more resources by the early 20th Century.
Then Japan sees the problems with empire-building and instead limits their efforts at power at influencing their neighbors rather than conquering them. This allows their alliance with Britain to remain intact and allows them to enter WWII on the Allied side. China, meanwhile, sees the Soviets strongly back Mao after the Long March, and so Chiang sides with the Axis. Over the 1930s, Japan plays the field in China for influence, but Chiang's joining the Axis leads to a full-scale Soviet intervention on Mao's side in 1945, using the battle-hardened Red Army to settle that fight. Chiang's forces are destroyed by mid-1946, Chiang himself is hanged in December 1946. Much more allied with the USSR, China is openly hostile towards Japan, and the Allies allow Japan to formally annex Formosa into Japan in 1947 to keep it from Mao and the Chinese communists. (This also allows Japan to keep Sakhalin Island and part of the Kuriles.) Japan sides entirely with the West in the fight to contain communism and fund a major armed forces, but being part of the Allied armies against Hitler allows the Japanese to learn just how much has changed among people, which the Japanese proudly add to by pointing out that they had not discriminated against Koreans or Formosans in years.
The result is that the current Japan includes Korea and Formosa, as well as Sakhalin and parts of the Kuriles, and Japan's population is nearer to 200 million, and the country is one of the world's major economic powers.