How about this : long-distance recruitment.
Since your clan can have multiple parties, and even caravans, it would make sense to expand those functions to allow your companions to recruit for you in your heartlands, and meet you up at the frontier with freshly recruited national troops. Indeed, I sense the whole recruitment options available to the player will be expanded in the future.
For instance, since villagers spawning from villages now actually carry goods into towns, and recruits are actual "units" now, shared between you and all AI lords, it would make sense to have your castles summon small parties of recruits from it's associated villages from time to time to add to the garrison and train into proper soldiers. From there, companion parties could be tasked to bring troops to you, transfer troops between fiefs, or whatever, really : they sky's the limit with Bannerlord's new system, i doubt they'll limit themselves to Warband's style of running around from village like a beggar deep into the mid-late game.
As for culture conversion, I think it makes sense, and one of the best way to implement it in my opinion would be with the option of adding new characters (therefore recruiters) to conquered villages and towns, a mixture of settlers and governors of sorts, who would offer you troops of your kingdom's culture. They could even have their own quests and everything, so they gradually "open up" their troop offerings the more quests you do for them, though the native characters of a village might hate you for it