I think the most likely route at the moment is schism and fragmentation within the existing power structure along geographic lines, as happened with the Soviet Union. I suspect this would only be possible in the face of sever economic crisis, in which the economic and political interests of the regions severely diverged from those of the central authorities.

It's difficult to see this actually happening though. China is pretty prosperous and can probably weather even a severe economic crisis. The Soviet Union was far more ethnically and regionally diverse. It also had the Eastern Bloc countries under a loose enough control that it was possible for them to slip out of it's fingers, which started a domino effect that China isn't really vulnerable to in that way.

Russia was also hilariously poor, even when it was relatively powerful. Saying Russia is more ethnically diverse than China is an interesting statement, given the enormous breadth and width of China and all the different subcultures and people living there. Unless this is a skin colour thing? I don't really understand ethnocentric reasoning

Ethnic politics in China is like everything else a complete stitch-up. China, though big, is a fraction of the size of the USSR, which was 70% Slavs while China is 91% Han. That's less than a third as much ethnic diversity. The problem is a lot of even that diversity is fake.

There are some powerful members of the ruling Chinese elite that 'belong to minorities', but the threshold for that is having one ethnic minority grandparent, who themselves might have had only one ethnic minority grandparent. Oh, and ethnic minority groups get some educational advantages, so it's an advantage to get the classification. This means the ranks of minorities get heavily packed with people who are in practice indistinguishable genetically and culturally from the average Han, but have the right to represent and speak for their minority group.

Thus in theory minorities are very well represented, get various advantages and have a voice in politics and society. In practice the people who get to do so are hand picked by the party and have the most tenuous links imaginable to the minority they supposedly represent.

Leaving aside the frankly problematic amount of veiled racism, China is large. There are a lot of people in it so even if 91% is Han, they might still have more other people in it than Russia does.