I expect some people have different ideas but essentially: split it up into administrative divisions. Not totally historical, but an argument could be made there too- China didn't have the same style of bureaucracy of the later game Early Modern empires, so it could represent a relative level of decentralization different from what the game normally represents. Then the divisions + emperor are under an HRE style system where they can't attack each other.
Then it's the emperor's job to pass centralization reforms to centralize China, just like the HRE reforms, where at the end it gets to absorb the administrative divisions and turn into a conventional unified monarchy.
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that's the gist of it, but if you'll forgive me rambling, I've given this a bit of thought (keep in mind other people have lots of other totally different and potentially better ideas too):
For my idea, Ming starts at a middle level of centralization, where the divisions can't attack each other. Based on filling out a bar like Imperial Authority in the HRE (call it Mandate of Heaven!), the centralization level can go up, to unification at the top- but also down, to a point where the divisions
can attack each other, and finally where they get total independence at the lowest level (there could be a lot more levels in between though).
How you get more (or less) Mandate to pass reforms with could be on the one hand out of the player's control, from all sorts of things- events, stability, legitimacy. If you lose in a war, you take a big hit, and so on.
But on the other hand, there's another part where it is in your control, and this would be the gist of what you'd do when playing China- interacting with your neighbors by expanding, and maintaining, a tributary system.
See I think one of the central things this system should accomplish is to stop China from being able to blob all over Asia- but it's really hard to do that without also taking out war, which makes for an extremely boring game. So with this idea, rather than wars of conquest where you take land, you as China go on wars to bring neighbors into your tributary system. At significant warscore (maybe even 100%), you get to offer a peace deal where they become a tributary, at which point they contribute a slow tick to your Mandate of Heaven bar. How much would depend on their size- subjugating Hsenwi and Ryukyu for instance could give you a tick so small it would take centuries to pass a single reform. But if you added in Korea, Japan, and Mongolia, you could maybe do it in a couple of decades or so (or it could take way more than that, these are just loose numbers).
So by warring and adding neighbors to your tributary system, you can pass reforms and centralize China. Directly taking land should be banned, or at least prohibitive- that's half the point of the system; say for every province you take directly, you lose 10% from your Mandate of Heaven bar? (this way you could still neaten up your borders but not go wild). To keep it from having you do mega-warring for the first reform you pass and then just sitting there after as the Mandate bar passively ticks up, maybe all subjects break free each time you centralize (.. sickened by your abandonment of traditional Confucian values?

I'm sure you could find a historical justification)- or maybe it just takes way way more subjects to pass the later reforms. Alternatively you could just have each subject grant a lump sum to your Mandate upfront and nothing thereafter.
A couple of other notes: tributaries should give Mandate, maybe trade power, but nothing else. Chinese tributaries didn't actually give one-sided tribute, it was a mutual thing- they would send a mission to China, China would give them something in return. Often (as in Korea's case a lot) what they got in return was actually more valuable than what they gave.
Also, obviously I think banning tributaries from fighting each other would make the East Asian game extremely stagnant, so instead of that I'd say something like... China gets a big mandate drain for tributaries at war with each other? It can resolve this by intervening in the defender's side, or when one of its tributaries is attacked (like in the Imjin War). Naturally being World-Police and intervening in all these conflicts should give a China player something to do.
I hope I explained that well enough, it's been floating around my head for a while. At the end of the day I'm sure there are lots of good solutions, some that don't involve splitting up China- but I really think there needs to be
something that achieves the Sino-centric world that East Asia was in the period, stops Ming from blobbing like crazy, and lets you give them their historical power without making them OP. I think this does all 3. Thanks for reading!