After Putin, the oligarchs will scrabble about trying to get the most of what's left. If it gets really bad Russia will descend into civil war. Russia just recently had a mini-civil war with Prigozhin's rebellion.
If Russia ends up in civil war then China will invade Siberia under the guise of peace keeping to secure the resources it currently depends on from Russia.
China will cut deals with whichever oligarchs win out and part of the deal will be that China takes territory that they see as historically Chinese (such as Outer Manchuria).
https://www.rfa.org/english/opinions/2024/11/23/opinion-myan...
https://theconversation.com/chinese-security-companies-are-p...
https://www.voanews.com/a/china-backed-election-raises-fears...
If Russia decides to have a civil war, China will do the same in Russia.
China will also not invade Siberia, it's much easier to control it economically than turning your quasi-ally into a mortal enemy. (besides, nukes)
Or you have a military weakened and degraded by a pointless war of choice in Ukraine.
And you have many armies controlled by oligarchs and warlords looking after their own interests (like Prigozhin).
And you have many separatist movements, such has for example a movement for Siberian independence.
And you have a power vacuum after the head of a mafia-style state is deposed or dies.
After Putin all bets are off.
Exactly this is happening in Myanmar right now: https://news.ycombinator.com/item?id=42572620