Someone said this before, and I don't know if you're considering it, but how about Korea?
You can look at my timeline for some ideas (although I do agree that it is a bit ridiculous, it's quite plausible if you consider all of the intricacies), but if Goguryeo, under Gwanggaeto, had managed to exploit the Chinese when they were as divided as they could be, expect for probably the Warring States Period, then I'm pretty sure that at the least, you could see Korea uniting East Asia for a while, even possibly eliminating Japan as a country, and the history of the region would be drastically different.
In order to do this, though, you'll need a POD of late 300's to early 400's. The next plausible time that Korea would have become more powerful than OTL would be around 1400-1600, when the Ming and the Qing struggled to gain control of China. However, Korea would only have influence over southern Manchuria, and I'm not sure if their claims would hold out if Japan managed to dominate them in the same way as OTL.
Just throwing some ideas out there.