Countries releasing unreasonable vassals

[Vohen]

Giving extra gov capacity won't fix, as then the ai just expands even more

There's very little we could do to actually fix the problem through modding, as it's due to hardcoded AI behavior afaik.
So, at the very least, until the next patch comes who knows when, our hands are tied for an actual fix.

At most, what we can do is eliminate the root of the problem for the AI as a bandaid until then, that is, since we can't help it managing their GC, we can make them not have to at all.
I agree that 100 or 200 would just be a slight delay in eventually suffering the same issue, but going completely overkill with a flat 5k or 10k increase would virtually eliminate GC as a mechanic, and I doubt we'd ever see the AI releasing a vassal due to the lack of GC.
Mind you, I'll reiterate that it's just for the AI, and a way for players themselves to do something about it until devs can come up with an actual fix for this.

How about making vassals and releasing nations instead of conquering land when AI is close to 100% instead of releasing vassals after passing GC limit? I know, there will be issues after Iberian Wedding or Inheriting Burgundy, but this could be "fixed" with some additional, maybe temporal, modifier to GC after certain events.

As an actual fix, for devs working on AI behavior, I figure it'd be better to simply have the AI do what's actually logical, like a player would do.
Vassalizing instead of conquering is one thing, but also, turning states into territories (with priority to high autonomy ones), building courthouses, and never integrating vassals, just have the AI focus on doing that whenever close to the cap and the issue would be solved.