Might as well post this here too:
Also, I did a bit of research, and a Japanese faction would be seriously outgunned by the Western faction. Basically, the Japanese army remained the way it was during for example Shogun 2, until the modernizations in 1860's and forward. Therefore the Japs would have to quite literally fight muskets with swords, bows, spears and matchlocks.
On the upside, it shouldnt be hard to find a mod that has all the required armor and weapons and ask for their permission to use them in the mod.
So basically, if any of you other old guard remember the patch for Musket Madness where the Samurai faction got added, thats exactly what it would be like.