I think you're giving Shogun too much credit for diversity; there are minor differences between factions that matter in grand strategy, but every battle plays out much the same, regardless of factions involved. I think a Warring States game could work in the same vein as Shogun or the Rome games that rely on control of the senate, with people competing to control the Emperor. If the exoticize the units a bits (with mercenaries from the steppes in the north, Southeast Asia in the south, etc.) you could introduce a bit more variety.
And hey, Koei is open to partnerships; maybe make a Total War/Dynasty Warriors mash-up, with general units returning to the heights of Medieval II, when they could slaughter small armies by themselves.
OT: Looks kinda interesting, but I'll delay until I see more. Arcane Thinker mentioned the Barbarian Invasion expansion for the original Rome, which had good ideas but executed them kind of poorly; if they can show they've polished things like wandering hordes on the strategy map, I'll definitely give it a buy.