New Types of Diplomacy

I think there should be more complex options in diplomacy, like vassalization, and making someones land a province, or shared territory (condominium). Along with this, though it may seem unrelated, there should be a way to send other players units, so the gameplay becomes more tactical, which would go along with the complex diplomacy because this could be included in types of deals, like vassalization for military protection/ sending units etc.

Aren't those just alliances? I mean... an alliance can be applied as a vassalization pretty well right? And just steal his land, that way you can "share" it. And as an ally you can send units as well.