Although diplomacy is improved in comparison to previous versions, there are still some things that I don't like.
I think the creators need to overhaul the entire concept them have about states running diplomacy. Before the rise of International Law, diplomacy was somewhat different in practice. Unlike today, states in the past were much more liberal in entering war and concluding peace. In Total War, much less though in Shogun 2, states remain enemies for the duration of the campaign after one hostile event. I think the creators such change that, making it possible to conquer a province from another state and become allies the next turn. Of course not all factions should be like that, but certainly a substantial number of states should behave like that.