China invade Russia? That would be a curve ball. I know you didn’t suggest it btw.
Well, taking the WWII analogy on, if Putin took Ukraine and continued, eventually resulting in actual military intervention from the West, Putin would have to look to the east, China specifically, for the kind of military support he would need (just as Hitler looked to Russia).
If China said no (as they would likely want to wait 'til the last moment to save themselves money and ensure being on the side of the victors) then Putin's deranged state would probably see him take that as them being the enemy and needing to fight them too.
So I'd say eventually it'd be more likely Russia would invade China than the other way round.
I know how ridiculous and suicidal that sounds, but people said exactly the same about Hitler and Russia. By that point he was so drunk with power and nobody dared tell him otherwise that this invasion, which almost nobody apart from himself thought could succeed, happened.