Personally, I think all you have to do is wait. China has a history of periodically falling apart into contending states, and just as much of a history when it comes to 'falling back together again'. Granted, the current period of division has been going on for quite some time... but who knows? A hundred years from now, we might well be looking at a new period of Chinese unification!
Anyway, to prevent the current period of division the best POD would be late-Ming reforms, but the OP rules that out. Second best option: somehow unite the so-called 'Ming Remnants' that held on in the south. In certain historiographical traditions, they are even called the "Southern Ming", but that's mostly just a name without a concrete entity to back it up. The successors to the Ming were mostly warlords who squabbled over legitimacy and only presented something of a united front to keep Li Zicheng from invading the south. End result: multiple states in what used to be South China.
And then there's the Xi Dynasty, established by Zhang Xianzhong in Sichuan...
If the Ming Remnants could have united under one leader, thry may have been able to wrest control of the north back from Li Zicheng, before deposing Zhang Xianzhong and thus re-establishing order.