Historically, Japan had such an inferiority complex they actually considered mating japanese women with Gaijin europeans in the belief it would improve the race- mind you this was in the 19th century where nationalist racialist 'race science' was all the rage. THe japanese archopelago doesn't need such people, but the land would be a fine addition to China proper in the same manner the lands of the khitans became part of China proper. In fact, Song China had cause for grievance against the Khitans for their version of the Rape of Nanjing in the 'JingKang Incident'. Now, there are no more Khitans, but there is China.Japan yearns for decolonization and denazification. In ww2, they ironically projected their own mental colonization to claim they would free Asia from imperialism, when in truth, what was always needed was for Japan itself to be set straight and returned back to it's family.
All of Japan's traditional roots originate from the mainland. When they tried rejecting it in favor of aping the west, the results were laughable and turned them into a soulless nation.
China should use all means, diplomatic, economic and military, to recover Japan as its subject, which is the role that will benefit the Japanese people most in the long run.
I do believe we are witnessing the modern day equivalent of Rome abandoning Roman Britannia, complete with the attempt by Rome (Anglo led West) coping with manpower shortages in the army by promoting mass immigration (barbarian foederati) along with rebellious provinces (Texas).