A few ideas to make social part of the game more interesting.
1. Introduce Kingdom goals. Kingdom goals are set by the King of a Kingdom, this goals can be predefined (e.g. accumulate 5 million gold or constructing some kind of monument), basically something that requires a certain level of unity within a Kingdom to accomplish.
2. Accomplishing the Kingdom goal translates to huge rewards to the King. The King will also be able to dispense/share the rewards with his subjects once the goal is reached.
3. Give the King some form of power over his subjects, nothing permanent but definitely felt by whoever is being punished. This allows the King to "unite" his subjects to accomplish the goal he has set. Of course doing so may actually "unite" players against a tyrannic King. This makes playing as a King a balance between carrot and stick to gather support for his goal.
4. If a subject has enough power (influence?) in game, he may lessen the punishment a King may be able to place on him. Realistic to a certain level as it is easier to punish a commoner compared to a powerful Duke.
5. After winning a war, if the winner is the attacker, the attacking King may delay the losing Kingdom's Kingdom goal in some form.
Basically I think adding some form of goal be it long or short term and more interaction between players will improve the social component of the game.. who knows you may end up having some disgruntled subject/faction working with another Kingdom to sabotage his own King's Kingdom Goal.