Add option to propose vassalized nation help in their way to freedom, maybe even in different stages as:
- Hidden declaration of help (when this nation rebel against their masters you will be asked to join them in war, no negative impact with suzerain before that)
- Open declaration of help (when this nation rebel against their masters you will be mandatory to join them in war, negative impact with suzerain before that)

Maybe even some diplomatic or espionage system to discover hidden declarations or push to revoke it.

In different manner I would sometimes like to vassalize vassals of my enemy, now I can only conquer vassal cities and in war menu vassalize my enemy or capture vassal cities.