How to keep the late game interesting

only warrior dont turn on their master but non warrior like smuriai will !

when it comes down to it i think historical accuracy could be sacificed to a certain extent if it meant better gameplay, keep lots of clans with 1 well-developed region each maybe (even if they werent historically around at that time) and the big clan could hav undeveloped countries

another idea, add clan specific japanese units in the late one, mostly for the clans with a disadvantage, though that would mean more work for you, anyway im sure the 70% domination could be scaled down to 50% without too many noticing ;)

I agree that there is a need to make it more difficult near the end, however, you don't want it to be so annoying that you just couldn't be bothered to finish the game. Maybe a bit more corruption to constrain revenue. The old STW "re emergence of faction" perhaps, if that can be arranged, as long as the armies aren't too large, since you can't teleport (ship) your armies the length of the country any more.

What you don't want is piles of extremely annoying rebels popping up all over the place, and you spending most of your time getting rid of them. Especially since there is so much forest, they could hide out there for decades and not be noticed.

Sadly, what I wanted to do could be done in MIITW, namely open up Korea -> Beijing as a place to invade just for fun. Expand the map on the year when Toyotomi Hideyoshi. I would love to fight upstream against the Ming spam.