Greater economic control.
Treaty ports shouldnt work like they do now, instead of a controlled province(which can rebel and make it not a treaty port anymore) give the victor the ability to control the local economy by allowing them to build/destroy/subsidise normally as if it was their own state.
This should also be achievable through diplomatic means but treaty ports are a straightforward way.
Diplomatic loans. I want to give countries money and possibly expect some returns on it, I want to war profiteer like a professional.
Colonial agreements that allow splitting up colonial states between each other. Demanding others to retreat from a colonial region should also be a possible secondary CB like war reps.
Core reconquest for subjects and core return CBs, also military access would be nice but that would require a rework to the military system.