One thing that would be interesting, but would probably end up being way too in-depth for the game, is that resources are generated on individual estates. The estate holder would have an oath to deliver a certain portion of those resources to the lord. Who would then have an oath with the duke to deliver certain portions of that to the duke, and the duke to the crown. i.e. a knight has an estate the generates 100 bushels of food, 100 units of stone, and 100 gold. His oath to his lord says "You owe me 60% of everything". So on each tax day the knight gets 40 gold/food/stone, and gives 60 of each to the lord. The lord collects everything from his own estate, plus the estates of his knights, and then delivers his own percentage of that to the duke. The lord/knight could then negotiate the percentages of each. The knight could say "I don't want the stone, but I want the food to supply my trading ventures" and the lord could say "I want the stone, so I can send it to the duke for his walls, you can keep the food", etc.
While this would be interesting, I think it might be too detailed to fit into the game.