If you want to increase China's landmass and take back lost lands, just invade Mongolia.
I oppose this idea because you're creating a 20-year Iraq-style problem and a 100-year integration problem that distracts and delays Taiwanese reunification.
As for Russia - Russia borders Alaska, and Russia's southernmost Kuril Island is 8 kilometers away from mainland Hokkaido. Today Russia has military bases on the Kuril Islands and Sakhalin. This could be invaluable in twenty-five years.
It's always weird and even suspicious that SDF goes on and on about Outer Manchuria and rarely mentions Outer Mongolia, when the former gets you nuked and the latter just requires some long driving (ignoring the twenty-year insurgency that follows).