Your challenge is to reverse Japan and China's histories. China becomes an empire that strives to conquer Asia, while Japan is weak, that's invaded by China, and one that is threatened by communist rebels, which takeover the country.
To be honest, all you need to do for that is to make China industrialize successfully - and for that it's really a question of stronger late Qing leadership (they came pretty close in the 1860s).
This is because nationalism/chauvinism seems to be a rather natural outgrowth of 19th century industrialization, so assuming a successful implementation of 'Qing-ism' there will always be perceived humiliations to be avenged and 'natural boundaries' to be reached; which in turn means an almost inevitable expansion of China.
Similarly, an industrialized China is going to be able to beat an industrialized Japan down in a dispute over Korea. This would dramatically destabilize Japan's politics, which was already quite threatened (or perceived itself to be threatened) by anarchism/socialism in the early 1900s. A few lucky strokes and the monarchy could be brought down, ushering in an age of general instability which would enlarge the Japanese Communists even more than OTL... then it's just a matter of time before Chinese politicians step in to 'restore order' or something.