Only when Russia has been totally nuclear-disarmed that China would ever consider such possibility of grabbing Siberia and the Russian Far East from Russia.
However, even if China really do want to retribute against Russia for their participation in the Eight-Nation Alliance 122 years ago, there's literally little natural resources and agricultural land that could be offered to China - Apart from expanded land for China to manage, and expanded coastline for China to guard.
China might as well annex Outer Mongolia and reforest their portion of the Gobi Desert for climate improvement and agriculture, plus fully securing the supply of coal for the next few decades.
Which is why there are so few population centers i.e. cities across most of Siberia and the Russian Far East, simply because there isn't enough aerable land + suitable weather and climate that could sustain permanent living on a large scale.
Even if further climate change results in those permafrost lands in Siberia and Russian Far East to become suitable for agriculture again, how long should China wait till then?
Apart from, say, Khabarovsk and Vladivostok regions plus Sakhalin Islands that historically do belong to China's Qing Dynasty before being forcefully ceeded to Russia, I don't see how China taking over Siberia and the Russian Far East would be benificial to China's cause and interests.
