Originally I didn't mean to make this so long but I just started thinking about how the game could be better in this aspect.

I definitely thought there should be an expansion here as well. Vassals just feel so hollow since you cannot really do anything with them outside of it guarantees they wont go to war with you. I also have no clue if they are actually fighting/helping me in war since I really cannot tell who is fighting battles outside of random movement of AI armies. Some things I would like to see are

1. Vassals joining your battles as reinforcements because they are nearby or just seeing if they are in battle also being able to join their fights

2. Being able to set war goals for them IE ransack these territory or territories, hunt down enemy armies here, defend these territories etc.

3. I should be able to reward them lands from wars 

4. vassals not being able to become the religious head of the religion you started. While I like the idea of religion being able to change heads similar to the origins of Christianity in the middle east than proceeding to be headed in Rome. It would not make sense for vassals to take control of a religion when you are in control of their government.     

5. This will be the long post for sure but would really flesh things out. A new dynamic relation system with them. I should feel like I actually took over their government and same with losing to them. While genocide definitely happens, usually humans occupy and set up puppet governments or assimilate the conquered nation especially more so in the modern eras where everything is televised. This would incentivize keeping them alive instead of destroying an AI or player. After first vassalizing they should start out as a occupied regime where the player makes choices over a few turns on how the relationship between the two should be handled. Some of these options can be are they allowed settle new territories freely, do they need for you to approve, or are they not allowed to settle new territories at all, Do they have to follow your religion, do they have to have the same ideologies as you, how much they have to pay in taxes, etc. 

All of this should affect their occupation stability regions that border your nation and their capital will have stability bonus, while territories that are far away will have no such bonus thus more greatly affected by your choices. The harsher you penalize them the more it affects their stability, if stability is lower than a certain amount like half than a civil war should start. Where the unstable territories become rebels that want to reconquer their nation but should avoid leaving the borders of their overall nation to trot off some where randomly. While the stable territories are your allies in the war. If the occupied government are a player they should get the choice to become either the rebels or allied government. Players or AI that are not involved in all of this should not be able to declare war to conquer rebel territories since they would be inherently weak but they should be allowed to gift units or gold to fund them or even outright send spies or saboteurs as support elements without giving away who they are.

The occupation period should be a short amount of time like 5 turns or so, so this period would happen rather quickly and rapidly based off of the winners decisions. After the occupation period ends this the new government should form which starts to actually interact with the world again. You cannot rapidly change your choices/ rules you made for them but you should be able to slowly do it over time using the choice of influence or gold. So essentially raise their taxes on turn 5 pay either xxxx amount of influence or gold. You have to wait another 5 turns to change another rule like can they settle. Over time depending on how much they like or hate you will depend if they create barbarians, raiders, or terrorists depending on the era to disrupt your nation. This would incentivize having fair conditions with them.

If all of this was placed in the game it would slow its pace down drastically for war mongers since they could not just win and turn their armies around to the next nation.

5.Countries that have the liberty should be able to place a grievous as a demand after a certain period of time against you for puppeting another nation for long period of time. Liberty minded AI should incentivize puppeting over conquering you and also release you after a certain time when your ideology also liberty minded like them.