I think this needs more AI scripting and personality/mindset modelling. Existing guidance variables are: AI honor, AI greed and AI rationality. All of them should find more application to:
— claim fabrication
— pushing of vassal/courtier claims vs current incumbents
The way I see it, positive AI honor should prevent a ruler from using his chancellor to fabricate claims (except for claims the ruler believes he should have, maybe, e.g. claims his ancestors forgot to press). Positive AI honour should also typically prevent claims of younger brothers being pressed against elder brothers, claims of sons vs fathers etc. On the other hand, high AI honor should encourage the AI to attempt restoration of 'rightful rulers', i.e. deposed previous holders and their successors.
Without high AI greed, AI should simply not be interested in fabricating claims anyway.
High AI rationality should probably prevent AI from antisocial behaviour. I'm treating 'rationality' as something broader than strictly logical consistency in one's behaviour, more in line with the concept of being 'reasonable'. Reasonable rulers don't expand through claim fabrication or pressing of their vassals' and courtiers' claims against their dynastically senior relatives who are in charge of larger fiefs.
Now regarding the ease of expanding and staying expanded. Do you remember, perchance, the Netherlands from Europa Universalis? That was a high nationalist modifier resulting in something like 70% revolt risk. I think CK2 should have a higher revolt risk for everybody who was:
conquered, or:
inherited in a disputable way and misgoverning/mistreating his additional, non-primary domain
Being inherited by a foreign-speaking 4th cousin of the last native king really isn't the end of the world, if he's at least semi-decent. But when he starts acting up, he can't count on the same leniency, deference, reluctance to disobey/oppose etc. as people would feel with regard to a clear-cut native rightful ruler.'
The game's AI should 'learn' to make the difference between foreign conquerors and foreign inheritors. Foreign inheritors should face prejudice, but foreign conquerors (and their descendants throughout at least a couple of generations) should face extreme prejudice.
The game doesn't have the mechanics to reflect good or bad governance of the realm, so let's take maybe Crown Authority. Where the crown worn by a foreign conqueror or even a country cousin/foreign relative of the last king attempts to aggressively assert its powers, that should probably trigger much greater backlash than the usual curbing of the power of great nobles in favour of royal appointed magistrates.
I also think peasant revolts should in some cases support deposed monarchs and their offspring, especially people who had: Kind, Just, Charitable, Brave (in this order) and maybe those surnamed 'the Holy' or 'the Blessed' or 'the Great' or 'the Magnanimous', maybe 'the Wise'. Just because, let's say, 3 dukes and 5 counts aren't raising their banners doesn't mean the populace (wider masses of peasants, non-patrician burghers and minor nobles like knights) aren't itching to take up arms for the good old king vs the usurper.
