Well, we can start with the puppet governments established OTL; in which Japan established the puppet state of Manchukuo, and the officially autonomous (but in practice independent) state of Mengjiang; both were attempts to weaken China through a "divide and conquer" strategy. Both of them had obvious divisions, since both had Manchu and Mongol minorities. I say minorities because at this point, both countries were pretty thoroughly Sinicized, and the difference was effectively meaningless except for the purposes of propaganda.
The Japanese had established a puppet Chinese government that was officially the "real" government of China, and which nominally ruled all of China except for Manchukuo. As seen by Mengjiang, this was not intended to be the case at all, for the above reasons of control. It is likely that further balkanization would occur, probably along ethnic lines (the removed states may or may not be classed as "autonomous regions," probably depending on political expediency). Anything past Qinghai is really outside of Japanese interest; but you would probably see attempts to sever Xinjiang and Tibet from China, and possibly a division within China proper itself. Probably the most likely division would be north-south, with two separate puppet states divided by the Huai or Yangtze rivers. The end goal would probably be an attempt to destroy the unified Chinese identity, to prevent nationalism from throwing off the Japanese.
Chinese reunification is not inevitable; the various ethnic groups and potential power structures have no prime directive to set apart their differences for the glory of Tianxia.
Virtually nothing is inevitable, but by the 1900s, the unified Chinese identity was already as much a national identity as the German national identity, if not even more so. If you want to get really technical about it, the German states in the 1800s and 1900s had no prime directive to set apart their differences aside from the glory of Germany.
