I think the best model is small-ish democratic polities that collaborate closely with each other. The federal government should be relatively weak, acting as a backstop.
The US is a pretty good example of this. The EU is another. And within the EU, Germany would also qualify.
Russia and China are both imperialist empires that need to break up and democratise. The first step should be losing their colonial possessions. China should lose Tibet, East Turkestan, Outer Mongolia, Hong Kong, and Macau. Russia should lose the Finnish border areas (particularly Karelia), Tatarstan, Chechnya, and far more.
Even following that, those two countries would still be too large to function as democracies. They'd either need to grant very high levels of autonomy, or better, entirely break up. "European Russia" would be a large and prosperous state under good democratic governance. The Asian part of Russia has different priorities, and it's nuts that people from East Asia are being recruited to fight for European Russia to seize Ukraine for itself.
As for China, once the colonial possessions are stripped away then my (uninformed) view is that there are basically three regions with their own interests - the West, North East, and South East. Each of them should have their own foreign policy, defence policy, and so forth. I imagine SE China would become an "Asian tiger", trying to imitate Singapore on a larger scale. It's very connected to the wider Asia-Pacific region, and places like Shanghai, Shenzhen, and Guangzhou would benefit from more "openness".