I feel like it's only a matter of time before they add China and/or SE Asia to the game. Which would be awesome. You have the Tang Dynasty in China, the early cultural contact between China and Japan, and the An Lushan Rebellion.
Since I know you know a lot about China, how do you feel about the "the CK2 engine can't adequately depict Imperial China" argument?
Actually, I think it can handle it pretty well with a few tweaks. China was a classically feudal empire from at least the later Han (~25AD) forward. In a lot of ways, Chinese dynastic history strongly resembles CK2 blobbing and fragmentation. You'd have a solid blob of China (which was considerably smaller than modern China) for a couple hundred years, then either a dynastic revolt (similar to the Muslim decadence revolts), a foreign conquest (little harder to replicate, espeically the part when the invaders became Chinese rather than the other way around), or disintegration and chaos (easy enough).
Here's China's dynastic history during the CK2 timeframe:
618-908 Tang Dynasty
907-960 Five Dynasties and Ten Kingdoms Period -- similar to when the HRE is weak and most of the Dukes claim independence
960-1279 Song Dynasty -- established when one of the fragmentary states conquered all the others
960-1127 Northern Song -- conquered by the Jurchen Jin "dynasty", capital moves south from Kaifeng to Hangzhou
1127-1279 Southern Song -- Song imperial rule continues in the south until conquered by the Mongols under Kublai Khan.
1279-1368 Yuan Dynasty (Mongols) -- China ruled by Kublai Khan and descendents. Part of the Mongol Empire until the fracturing of the Hordes.
1688-1644 Ming Dynasty -- Han rule re-established through popular uprising.
I think they would have to add a few mechanics:
1.
Face/honor -- Kind of like decadence in that it would affect the whole family but there should be either a weaker ripple effect or seperate personal and family honor levels. Low enough personal levels would allow your liege to revoke your title for free. Low family levels could make you invalid for being invested with new titles. Plus a general malus for low honor. I suppose this could be done with prestige, but it's too easy to abuse prestige, and honor is somewhat different -- more like a mix of piety and prestige.
2.
Naval combat -- As pointed out, naval combat (a bit of a misnomer, since it was mostly river combat) should be a thing. Naval power is really the only thing that kept the Song dynasty alive as long as it did against the Mongols.
3.
Mandate of Heaven -- Stretching back into time immemorial is the concept of the Mandate of Heaven. Basically, if things are going good, the Emperor is assumed to have the blessing of the gods. If things are going bad (natural disasters, famine, plague, etc) then it's assumed this is the gods' way of saying "This guy sucks, get a new Emperor". Loss of the Mandate of Heaven often helped to magnify popular revolts.
4.
Resistance to culture change -- The Chinese are well-documented in their ability to culturally assimilate invaders, to the point where the Manchu emperor kept an entirely separate capital in Manchuria that all Manchu nobles and youth were required to spend part of the year at, learning and practicing things like horsemanship, archery and the Manchu language, lest they become Sinicized and "soft". Chinese cultures should be kept to two (Beifaren and Nanfaren) in the Han group, and be much harder to change with events.
5.
Tributaries -- Most, if not all, of China's neighbors were tributaries to the Empire when it was strong. When it was weak, it was often tributary itself -- especially to the steppe tribes of the north and west. Smart emperors played the tribes against each other to keep them busy and weak and allied to the Imperial Throne. Weak emperors were seen as an easy target by opportunistic nomads, who increased their tribute demands to burdensome levels. IIRC, tributaries are being added in the next expansion anyways.