I reckon stuff like kingdom alliances would be a cool feature, probably not that hard to implement either. Armies formed of two colors would be neat, a system of contract between the allies that affects how land is divided based on the leaders' relations with each other plus the traits they have. Contracts could be dynamic too, with multiple options like just a contract for non-aggression or purely defense (no multi-color armies just parties coming to aid when being sieged). The contracts would be made with an expiry time, followed by a cooldown that should block wars between the former-allies for at least a bit. An actual system like this would require a bit more thought than what I've wrote, it could just make things worse but more diplomacy options should be in the game and this, to me, seems like something that wouldn't be the most difficult thing to add.