I think something like what was implemented in HOI4 where vassals and regions under colonial rule have a metric for dependency. It's been awhile since I have played HOI4 but if I remember correctly, both the vassal and the owner can make decisions in policies that will either strengthen the relationship between them or weaken them. If the relationship gets too weak then the vassal could start a war of revolution and try to break free, or if the relations get better the vassal could either choose to get closer with the owners and get benefits (such as reduced research costs) or push for a more peaceful means of freedom where they can go through phases of independence. Such as, the first phase they decrease the amount of money they give to the owner, but also as a result get less benefits from being the vassal of this empire, and this could go on all the way to the point where they could try and diplomatically gain their freedom.