I'm surprised the Modding community hasn't already done something like this. But I'm equally shocked Paradox hasn't addressed this. Like, Utopia was a step towards it, but it was all internal and again, way too easy. And then they haven't touched diplomacy since. Despite the fact that I've seen it come up every time they release a patch!
Meanwhile Galciv III had an expansion pack all about Intrigue and Governments and interactions between different governments and gave you the ability to create client states and it's really fucking good. I just conquered the Kryyn, they surrendered to the Onyx Hive, and I had too many planets for my republic so I broke off the old Kryyn worlds and made the Kryyn Republic a thing. It was really cool, and immersive, and there were a lot of interesting ways it could have gone - maybe the Kryyn surrender to my other enemies, the Drengin? Maybe they didn't surrender at all and I conquer them world by world, overextending and tanking my morale? Maybe they surrender to someone who isn't on the other side of the galaxy? All this shit matters, and it can happen.
Meanwhile Stellaris allows you little customization of any states you create, doesn't allow the A.I. to spitefully fuck you by surrendering to your ally/frenemy, Governments barely matter at all, and gives the A.I. nations literally no flavor because they're always randomly generated. I like Stellaris, it's pretty and very fun in the early game before the doomstacking begins because it feels adventurous and dangerous poking around where there are big scary space monsters and aliens that might eat you, but once you've met one type of A.I. you've met it a hundred times because shit never changes. The only thing that ever prevents you from befriending everyone you meet is some of the A.I. nations are fanatics who are always at war with everyone, but then they're usually kicked into the dirt. There's no reason to want to be friends with everyone though, because there's no benefit to it. There's no reward for having allies. They don't do anything for you that just conquering their planets wouldn't do. It's purely a RP decision.
The Galciv A.I. isn't really all that smart, it just has personality. The restricted set of races means they can code specifically for that race, and then there's certain situations where the A.I. is programmed to stop looking for the best way to win and start looking for the best way to fuck the player as it loses. The Stellaris A.I. does not and cannot do that. For every nation, in every circumstance, the Stellaris A.I's sole goal is the same 4X paradigm it started with. It never goes out of it's way to screw you or another A.I., it never self-sabotages for petty reasons, it's never honestly petty at all. That's why it feels like Galciv's A.I. is better, because Galciv's A.I. is petty as fuck.