Well some research could be put into it, but beyond that I don't know how it could be implemented after the realm gets created. For the vlachs I think byzantine mercenaries could also be considered as they were in that area of influence quite a while, if an entirely new troop tree can't be put in once the game's on the way.
The quest could be taken from a potential lord as pretender, but not from one of the existing factions. Or from a village elder, case in which the player himself would be trying to acquire the title of King, prince or whatever. The ending could be a battle the player needs to win against the current owners of the land, after previously having to gather support from villagers and possibly the opponents of the faction he goes against to gain independence.
I don't know if vassalage is currently used in the game, as in instead of completely destroying a faction an option might be given to force its leader to submit an oath of vassalage to the victorious king and subsequently pay tribute and offer military aid in case of war. Similarly a quest to regain independence could be added for the player to take.