Hey Lukas , the vassals now are really boring you can`t do anything with them just ask him for money and that`s all.
Here I have some suggestions for them:
The ability to assimilate his population. Once the population of the vassals provinces is in minority you can make a referendum and the provinces join to you. The ability to ask from vassals some army.(like 0.5% of his population per round). When we are into a war every province conquered by my vassals to give me points not to him. The ability to change type of his government.
Thank you for reading.
◆Colony - ex. British Malaya, Viceroyalty of New Spain, Great Korean Empire
・The suzerain absorbs 50% of income of its colony.・The suzerain can recruit units in provinces of its colony.
・The suzerain can build something in provinces of its colony. ・The colony can't declare war.
This kind of vassal is created by annexation of tribal civilizations. The colony has some liberty desire when created.
the suzerain becomes to be able to annex its colony after 30 turns.
◆Subject - ex. vassals of Seljuk empire, Soviet Socialist Republic in USSR
・same as colony.
The suzerain can ask its subject to abandon its autonomy. If it accept, the suzerain occupy its provinces.
◆Puppet - ex. British raj, Holstein
・same as colony, but its suzerain absorbs 30% of income of its puppet.
If you win the war and create a vassal, the created vassal will become this kind of vassal.
The puppet may ask its suzerain to change itself to this kind of vassal when it has moderate liberty desire (20~30%).
Suzerain can deny its request but the war will be started.
◆Dominion - ex. Dominion of Canada, Australia(WW2)
・same as Vassal in AoC2 now.
(the suzerain can absorb only a few gold. The dominion can declare war.)
The dominion may ask its suzerain to release itself. Release or War.