What about some PoD revolving around the Anhui clique?
In the first decade of the warlord period, they were able to win many battles because thanks to their Japanese-supplied loans, arms, and advisor/trainers. They really shouldn't have lost to the Zhili clique. That they were defeated is, I think, due to the fact that they were demoralized and deletigimized after the May 4th Movement. So we need a way to keep that from happening.
My suggestion would be that the at Versailles, Japan receives only economic rights in Shandong, and the "sovereignty" of the territory is returned to China. This is what the Japanese Empire accepted in 1922, anyway, so I think they would (quite reluctantly) take it. That should cut the heart out of the May 4th Movement. From there, the ideological background of the Anhui clique's fall from grace, and the rise of the KMT, is mostly butterflied.
The Anhui clique has by far the best military, so they unify or near-unify China (the same way that the KMT often did: by getting other warlords alongside, not by conquest). They are something of a Japanese puppet, but not at all like in the Second Sino-Japanese War. They simply pledge to maintain Japanese economic dominance in Manchuria, Shandong, and maybe the northern plains area, and in return they get to rule over China. Japan doesn't have an interest in expanding their control further, since they don't want to risk what they've got. You would probably even see Japanese military assistance against the Communist forces.
There, Japan and China are allies.
Honestly, I think what happened in OTL is a bit unlikely. Japan saw China as simultaneously weak and divided (and therefore ripe to be conquered), but at the same time on the path to unifying and becoming threatening, especially after Chiang marched north and tried to take over Shandong. Push China a little bit in either direction, and all-out war becomes unlikely.