since it is possible to not solve a quest yourself but send a companion away to do it for you, you should also be able to send a companion in your party away for reinforcements from your culture of preference. maybe the game can check how far the closest settlement of that culture is away and then return after a few days dependign on distance with a number of recruits.
also, they should increase the role of mercenaries. right now, as a khuzaite, it is really hard to maintain a horse archer army. you only have khuzaites, the mamlukes (after tier 3 mamluk regular) and the bucellarii (tier 5 archer? and not even mounted before that). the only mercenaries you can recruit right now have a 'western' appearance and don't really fit for khuzaites due to chainmail etc. you cannot even recruit horse archers in taverns, even though horse archers were very common mercenaries in the middle ages. i don't think i ever saw mercenaries equipped with javelins either, whether mounted or on foot.
anyways, now with notables randomly disappearing, i think they should respawn eventually with a notable of the owner's culture later. i'm also considering if it makes sense for notables to move from one town to another like companions tend to do, but then towns in newly conquered regions would get crowded and the towns in the 'heartlands' empty too much?
No cultural conversions.
If anything there should actually be more penalties to mixing troop cultures -- and an option in Leadership to reduce those penalties.
historically that wasn't much of an issue until you fought the same culture. but one thign i saw recently rubbed me the wrogn way. i saw an aserai lord deep in khuzaite lands raiding a village, then they made peace and he returned to his lines and recruited khuzaites from villages along the way without losing a beat...
i just think you shouldn't be able to recruit from enemies and neutral villages as easily. mercenaries yes, sure. but not actual rival factions. raiding should cause a significant realtionship penalty across the board with that faction.