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What's more interesting to me is what you describe as a problem, but which is actually a more accurate representation of feudal society and a dramatic shift from Warband.
It stops being a war about Sturgia and Vlandia. Its a war brtween Clan Brugyudol and Clan Barthhearts being defeated by Clan Ulfru, they just happen to carry their faction banners.
This reduces faction identity too much as kings don't have much power and can't keep factions together especially if Clans are as greedy and backstabby as the blogs imply.
This is precisely what happened throughout history. Kings and emperors had to compel their vassals to cooperate with the crown, either subtly or overtly. Several kingdoms fell apart because of in-fighting of vassals which were selfishly trying to increase their own wealth, power, and influence. It is the king or emperor's responsibility to police their vassals, and the amount of authority each faction sovereign has in Bannerlord has been described. The Battanians, for instance, have a king with very little authority over the clans. The Imperials, on the other hand, fractured when there was no clear successor to their very powerful figurehead. The Battanian culture places very little importance on centralized power, while the Empire successor states consider it paramount.
Hopefully we will see these cultural differences at play in Bannerlord, where Battanians are effectively dependent on the whims of the most powerful clans (which are allowed to grow practically unchecked because the crown can't or won't stop them), while the Empire will have weaker individual clans but much greater cooperation in times of need.