Letting Japan keep Taiwan may be a good starting point for a more liberal (to some degree) and less xenophobic (to a larger degree) Japan. It would certainly make the Japanese less suspicious of their Chinese neighbours. I'm not sure how to make Japan keep Taiwan, but since China was still divided at the end of the war, there were many futures possible for Taiwan. Regardless, it all depends on how Japan would decide to proceed with governing Taiwan.
By the 1940's, schools in Taiwan were fully desegregated, and the Taiwanese were on their way to becoming accepted as Japanese. The Taiwanese had already shown by then through the Wushe and Ta-Pa-Ni incidents that they would not accept the suppression and discrimination originally planned for the colony. Many Taiwanese students were attending university in Japan and Taiwanese cinema was becoming popular in Japan.
Despite compulsory education in Japanese, there were still extensive underground movements of literature in Hokkien and Mandarin. Japanese-aboriginal interactions were mostly peaceful with extensive documentation of tribal culture and languages and have been since the Wushe incident, though the tribes were still exploited as soldiers and loggers/miners.
Having a large, more visible minority as part of Japanese society as well as a entirely different ethnic group that is regarded by the government as "good enough to be Japanese" should help pave the way for more acceptance of other ethnic groups. This all depends on how the Japanese government proceeds with assimilation of the Taiwanese. Since the Allies would not want any fervent nationalism from the newly defeated Japanese, the Taiwanese minority may be allowed more freedom to express their culture, producing a more heterogeneous Japan. There would be a larger proportion of Buddhists and Christians as well as folk religions and over ten million people speaking languages belonging to two entirely different language families. Japan lost many younger men as soldiers, while Taiwan came out of the war fairly unscathed. There could then potentially be a lot of marriages between Taiwanese men and Japanese women, further changing Japanese perceptions of what is "normal". Based on the current popularity of Taiwan as a vacation spot for Japanese tourists, there would be a lot of exposure in both directions as Taiwanese people go to Japan for education and jobs, while well-off Japanese travel to Taiwan for fun. Once again, what Japan decides to do with assimilating Taiwan would play a huge role in determining how much Japanese society changes.