Here's my idea, might be too complex though since M&B likes to simplify it.
Making a city, castle, or village accept you as their leader and thus offer their services, taxes, and recruits is more difficult than it appears to be.
First of all, it depends on the nature of the previous owner. If they were evil(lots of murder, fetch serf, make war quests), then the people would be happy to serve you in hopes that you were nice.
If the lord was unfriendly yet protected and provided, the people would be neutral and torn between their fairly good master and master that might treat them better.
If the lord was good natured, was friendly, protected his serfs and such, they would fiercely reject you.
So I propose a secondary relations number that is basically their friendship with you.
To successfully revert them to your faction requires a friendship rating of 100. Base number is -50, 0, and 50 for evil, neutral, and good respectively. For every 25 renown above the old lord, you get +1 friendship. You gain more friendship each day by helping your serfs and spending time with them, in the form of quests assigned through a new menu option.
"Assist the people."
These quests are random, and are completed in 12 hours or less based on skill level. Upon completion, you get +1 to +5 friendship.
Ironflesh: Play war with the young children wanting to be soldiers. (City or Village only)
Power Strike: Beat boiled hides for someone preparing leather armor or clothing. (City or Village only)
Power Throw: Play darts with the gang. (All settlements, night too)
Power Draw: Train novice archers. (City and Castle only)
Weapon Master: Perform tricks with your weapons for entertainment on slow days. (All settlements)
Shield: Train novices in shield use. (City and Castle only)
Athletics: Surprise folk musical dance! (City and Village only, night too)
Riding: Train novices in handling horses. (City and Castle only)
Horse Archery: Train novices for using bows while on horses. (City and Castle only)
Looting: Ornery kids have stolen goods, and you must retrieve them. (City and Village only, night too)
Trainer: Train novices for battle. (All settlements)
Tracking: Track down stray cattle. (City and Village only, night too)
Tactics: Instruct people to better defend themselves by using their surroundings. (All settlements)
Path-finding: Instruct a caravan master on which route is the fastest and safest. (City only, night too)
Spotting: Stand watch with the guards on the walls. (City and Castle only, night too)
Inventory Management: Organize a shopkeeper's inventory. (City only)
Wound Treatment: An old bowstring breaks and cuts the owner's face. You must treat the wound. (All settlements, night too)
Surgery: A horse from the horse merchant stall kicks the shopkeeper in the back of the head! You must save him.
(City only)
First Aid: Surprise building collapse! Forcing you to take care of the wounded. (All settlements, night too)
Engineer: Assist in construction. (All settlements)
Persuasion: Persuade Mr Jones to stop stepping on Mr Willington's cabbages. (Village only, night too)
Prisoner Management: Babysit the local children. Or, stand watch on the prison while the guard uses the latrine. (City and Castle only, night too)
Leadership: Become a motivational speaker to motivate everyone. (All settlements)
Trade: Take goods from one shop to the others. (City only, night too)