Paradox could just make another Segoku game that could include Korea and China. An eastern version of CK3 if you will.
That would be a pretty cool, IMO.
In regards to CK3, I would like to see China. I think Japan is too far (and again, they already have an IP based around Japan).
I think this argument of "you'd never interact with China" is weak. If I play Wessax I'm barely interacting with Africa let alone India as the game stands now.
The point wouldn't be someone else to interact with as some western European nation, but as more of the world to play. The Mongols interacted with them (hell, they conquered them). As I'm sure other nations currently in the game had interactions with them. Plus, it would open up a whole new exciting area of the world to play (much like India being added to CK series in CK2 expansion and CK3). China has a pretty interesting rich history that's worth exploring.