True, but the invasion actually had rather limited contact with Japanese forces. Two thirds of the invasion force were obliterated by a typhoon. The bigger problem is how to represent Japan, because even though Japan is rather small, it took a very long time for a centralized power to unite all of it and project military influence to other areas. So it's geographically small, yet to really prevent an early centralization of Japan, it would require many provinces. Of course Japan could be included with less provinces as a place mainly for the east to conquer, but that would be kind of lame. And frankly, in early medieval Japan, Japan as a whole was only beginning to formulate as a distinctive Chinese influenced culture that would eventually become what we would today recognize as Japan. Simulating Japan prior to the medieval period would be very hard unless Paradox just decides to make it a general feudal realm or general Chinese government type realm.
Also, with a China/Japan expansion, I hope we get a future expansion focusing on natural disasters as well, aside from epidemics that is. Floods, earthquakes, famines, that sort of stuff would be great for not only the east and the rest of the world as well.