I also liked Shogun 2's realm divide mechanic. Right around when you take over ~1/4 to 1/3 of the map, the Shogun declares your clan an enemy of the state and t

I also liked Shogun 2's realm divide mechanic. Right around when you take over ~1/4 to 1/3 of the map, the Shogun declares your clan an enemy of the state and the rest of Japan declares war on you after a few turns. It forces the player to bide their time, build up their forces, and think carefully how they'll plan their late game.