I still don't get why gradual cultural change of fiefs isn't part of the game. I'd even settle for auxiliary troops, like when taking an Empire town as Battania. The troops at that former Empire town will be Battanian with some Empire influences. This would apply to every land grab. This would however take much more time and effort as all possible combinations have to have full fletched troop trees (or maybe give the player the ability to customise such troops). Whereas fiefs converting to the owner culture, and thus the recruits completely as well, should not be that hard to implement.
I hate to see the circus of armies being a patchwork of troops from across Calradia. Prohibiting other kingdoms to recruit from fiefs that are not owned by their kingdom should also be standard (not for the player). This would've also been a fix for the endless roaming and recruiting by fiefless cockroach kingdoms,