The asian equalent of Medieval 2. This would mean china separated into many states, Mongols, Japan, Korea, Vietnam, burma, kambodia, thailand, malaysia, indonesia, possibly india on the far side. Yes it would be huge and yes it might be tricky balancing historical accuracy with the best mix of factions and technology level but lets face it, its far from impossible and quite doable as long as your target is not to put your audience into a juice press and squeezeeeee until every dime is gone.
No bullshit like the tiny ridicolous japan in shogun 2, or the epicly horrible failed scope and balance of empire but several factions in each country a true experience like the medieval 2 map but larger and more like the Stainless Steel mod. And for gods sake please abanding all the horrible mistakes youve been doing more or less since after Medieval 2. Most entries have a strong suit or two since then but overall its become horribly fixated on eye candy, arcady wannabe strategy and DLC. Release the entire game as a entity with every faction playable and if you want to make stuff after focus on proper expansions where you add big areas for example with the aim of eventualy including india, russia, middle east and possibly even europe. Real content not "added blood" or "2 faction x with a slightly different color". And make the preanimated duels a more rare kill animation so that units fight as actual units rather than one long que for repeated 1v1 duels.
Respect your customers and they will respect you back and return to the roots that made it a succesfull franchise.
Edit: Anything past the Napoleon era is not realistic and should not be attempted. It could be interesting but does not fit total war in any way and if it is done it should be a separate spin-off title where the necessary major adjustements on HUD, battles, units etc can be done to accomodate the new technology and era of warfare.