Thank you for the analysis, it is clear a LOT of work went into it!
My question is, what is the goal? What would be considered the optimal kingdom mechanics? I am genuinely interested, since I have not yet played a Bannerlord campaign past about 120 days, so I do have no idea what late-game feels like or what it should feel like.
I understand that one kingdom dominating most of the map after a few years is bad. On the other hand, if the map looks the same after 40 years as it did in the beginning, except for maybe 2-3 cities, is that the ideal outcome? Do players want the NPC kingdoms to be of equal strength, locked in a stalemate? Perhaps so that the consequences of player actions are the most visible (i.e. if the world changes significantly, the player knows it had to be his doing)?
Lets assume that is the case (i.e. the goal is for NPC kingdoms to remain roughly equal throughout time without player intervention). If that is achieved by kingdoms mostly trading one town and a few fortresses back and forth, so that the map looks roughly the same no matter at what time during the 40 year playthrough you look at it (this is a simplification of course, but I hope it illustrates the point), doesnt that make the world kind of boring? Again, maybe that is the desired goal, the world should be kind of "boring", or rather stable, without player interaction. But in my eyes, a more violent, changing world sounds more interesting. And I think it could be achieved even while keeping the primary goal of "factions are not eliminated and no faction ends up dominating the whole map forever". Meaning that wild swings happen, lots of territories change hands, but some kind of equilibrium is always restored to prevent a total collapse.
One way to achieve this could be some kind of kingdom management mechanic, where the larger a kingdom is, the more difficult it is to manage, the less stable it is and the more likely it is to break up, be ganged up on, etc. I believe Total War: Rome II had such a mechanic, where larger empires suffered greater and greater penalties (more internal political strife, more expensive upkeep of armies, more likely to be viewed negatively by others in diplomacy), so that they were attacked more, had more rebellions, and it was overall more difficult to keep the empire together.
Not only would this prevent snowballing of NPC kingdoms (violent swings could still happen, where a kingdom radically expands, but it would be unsustainable and it would break up, or be pushed back again), but it would also make late-game for players more interesting. As I said, I never got to late-game myself, but I read from others that once the player-owned kingdom reaches a certain point, the game turns into a boring, one-dimensional sequence of taking one fief after another. Making a larger kingdom more difficult to manage, defend and keep together would make the late-game more interesting (perhaps even shifting focus from constantly mopping up armies on the battlefield to proper kingdom management).
This mechanic could possibly (though questionable and perhaps not necessary) be paired with an opposite mechanic for very small kingdoms, making them even easier to manage and defend. Again, this would help NPC kingdoms bounce back, it would make it more difficult to completely finish off a kingdom (again making late-game more challenging). Also, it would make the start of a player-owned kingdom less challenging - of course if starting the one's own kingdom is already too easy, it would be counter-productive.
If I completely misunderstood the state of the game because I'm a noob, feel free to rip into me, I'm interested in all opinions and feedback.