It needs to be after the Soviets secured their political settlement internall (1922) and before Japan invades Manchuria (1931). Ideally, it would happen before the Nationalists begin building a real government after 1926.
Perhaps a scenario where Trotsky beats out Stalin early on (or something else that may cause Stalin or whoever is in charge be much less risk averse and prone to intervening), and in protest against the Nationalists turning on the Chinese Communists in 1926, the Soviets intervene in China? It's possible they might secure Inner Mongolia, Xinjiang, and even parts of Manchuria.
This is risky as a Soviet attempt to create SSRs in China would create all sorts of political problems.
1) It would be denounced by the League of Nations and allow its enemies to isolate the USSR politically. The last thing Moscow wants are the Western Democracies working with the Germans, Japanese, and other Fascists united against it. In case of war with Germany or Japan, the West will not intervene or stop it, and may even give support to them.
2) It likely destroys the the Communist Party in China, as most members leave in protest at Soviet (Russian) colonial designs in them. You loose much of the rank and file and even the patriotic Communists. You may have some hard core ideologues remain, as well as collaborating locals in the Chinese SSRs area, but as a mass political movement in the rest of China - it's dead.
3) The areas most likely to become an SSR are the least desirable - Xinjiang, Inner Mongolia, parts of nothern Manchuria. Small benefit compared to what did turn Communist later.
4) Soviets are now going directly against the Japanese. Japan will reorient its goals to stop Soviet expansion instead of growing "incidents" in China. In fact, Japan and the Nationalists might become full blown allies in order to stop the spread of the Soviet Union.
5) It would likely scupper any attempt at getting Western technology transfer to the Soviet Union. After FDR recognized the Soviet Union in 1933, various people emigrated to the Soviet Union (the poor fools), and several large companies entered into deals with Moscow to transfer plant and technology (relatively obsolete by US standards, but essential in creating a lot of Soviet industry). All of this is now in jeopardy as people in the US demand a stop to it in protest of Soviet actions in China.
