I'm also going to float the idea that, instead of having a setting with a Japan-analogue, a China-analogue, a Thai-analogue, etc. all side-by-side, you could just lump all of it together into one single setting: Shaolin monks and Taoist priests interacting daily with ninja and samurai. I suggest this because standard D&D has druids (Celtic origin), paladins (French origin), monks (some sort of Asian origin), bards (British/Gaelic origin), barbarians (ancient Greek origin, but popularized by Rome, and often applied to Germanic peoples), berserkers (Norse origin), etc. The races and monsters have similarly diverse origins -- it's basically a giant melting pot of European history and mythology, with the occasional element pulled in from more distant cultures (especially the monsters; certain really iconic creatures, like gold dragons and ogre magi and nagas, were Asian in origin). So I think you could do the same thing for an Asian-inspired setting: pull together all the common popular elements and just go hog-wild. It's not like China didn't have assassins who resembled ninjas, or that Japan didn't have martial arts monks.