Here's a few mods that add new subject types, if you're interested in adding compatibility. I'm sure others can add to the list as well, because these are just the ones that I played with and noticed in my last playthrough.

Galactic Imperium Unification - Adds a subject type called "Imperial Province" that Galactic Imperium members become along the path toward unification

Reworked Advanced Ascension - One of the ascension perks adds a new subject type, the "Personal Union", the name of which sounds nice for RPing feudal kingdoms