I have this idea, dunno if it would work as a RPC
You are the Worlds Defender of the Weak!
You have to be polite to everyone, trying to befriend all, however, should someone DOW a friend of yours, for rpc rules, a friend is considered a friend at pleased, you have to imidiadly come to the rescue.
If it is a friend of yours DOWing another friend of yours , you have to DOW the aggressor AND vassalize him/her, to keep them in a leash, since they have proven to be unstable for world peace.
Thus a Feudalism slingshot would help out tremendously to keep the Peace.
(For what its worth, I got DOWed in 750 BC by Julius in TMITs automatication worker try out

, so you have to be prepared for early vassals)
About Peacevassals, none can be accepted, that would make it too easy

However, if a war is ongoing, and a friend begs for your Protection, that is a whole different matter, you are after all Defender of the Weak
If it is a foe DOWing your friend, only complete annihialation is possible, to set an example for likeminded Villains.
Of course, should ANYONE be so stupid as to attack YOU, they are eternally doomed !
They need to be purged of the Landmass, and IF they have island/s , they are allowed as vassals, HOWEVER, their punishment for the uncalled for Attack on you, they are never allowed ANY resources whatsoever for the rest of the game, every time they get one, they need to imidiadly hand it over or face destruction.
My thoughts about who to play, is Julius of Rome, you may need the oomph of praets

, also the fast settlers, as you are not allowed to DOW first on anyone, EXCEPT aggressors (No early rush)
Just an idea, I know you played Chivalrous GW, but this has the opening to keep (even Boudica

) in a short leash
Kind regards from a long time lurker of your games
(btw, I apologize for incorrect gramma etc, english is not my first language

)