Maybe if faced with a strongly anti-Communist China Manchuria might be made a puppet. But the existence of a mostly friendly relationship with the Nationalists until WW2 and of a strong Communist insurrection under Mao, the Soviets opted to restrain themselves in China so as not to render their name mud with ordinary Chinese as just another imperialist power with a new hat. (They'd already passed up on the opportunity to annex Xingjiang and Mongolia in the interwar period.)