I recommend giving the Samurai Warlords Mod a try. It is a mod for MTW:VI and very well done. It is basically Shogun Total War with a bunch of improvements. I think that there are around 25 playable factions on a map of Japan with 60 provinces. The units have been modded so that they are the same as in STW too. Its one of the best MTW mods that I have played so far.