As a note on Japan's immigrant population, I have read that naturalized citizens, of which there are a fair number (Thousands of Zainichi Koreans naturalize every year, and intermarriage means a lot of Japanese probably have recent Korean and Taiwanese ancestors) are counted by most sources as ethnically Japanese, as the Japanese census only records nationality, not ancestry/ethnicity (Although they have their slightly overwrought family registration system, which I records naturalized citizens), so Japan is to some degree more multi-ethnic than can be accurately enumerated. An unknown number of Japanese have Ainu ancestry as well, and technically the Okinawans could be considered a separate ethnic group (Or multiple ethnic groups), so if you somehow boost Ainu and Okinawan national consciousness (Most likely requiring a pre-1900 POD) you could make Japan more diverse.
And as has been stated, if they keep ties to their former empire alive stronger, we could see more immigration from the former colonies, and they could, with no World War II or a somehow different settlement, keep Taiwan as an integral part of the home islands, automatically making the country more diverse with several million Chinese and nearly half a million aborigines. Aboriginal culture and identity may be stronger today if Japan still ruled Taiwan, as the RoC repressed them and they have a fairly fond view of the Japanese today, although this may be due to viewing the period through rose colored glasses.
During the post war economic boom, what was Japan's labor situation like? A lot of the diversity of Western Europe began due to the settlement of guest workers in the 1950s and 1960s to work jobs in the burgeoning industrial sector.