Add "colonialism, satellite states" mechanism

Also will be cool if there were some special treaties between vassal and liege that provide additional bonuses and penalties, so liege could choose between getting less benefit from vassal, but they benefit both empires, or get huge amount of benefit but oppress the vassal by penalties

I totally agree

Different subcategories of vassals and ways of vassalising cultures would be really interesting and add a lot of flavour to the game, maybe more war support is generated by more lucrative options for the liege, whereas less is generated by the less intrusive options, it could also make the placation of your vassal (with the diplomatic system from the expansion) an interesting way to mitigate smaller costs with regards to such a system.

Agreed. +1

Good idea. Only instead of specialized units it could be a bonus to stealth units. I also think vassals should only be able to research technologies their liege alread has. It doesn't make sense for a vassal to be more technologically advanced than their liege. Similarly lieges should get a tech boost to techs previously researched by their vassal.