New lords that enter the realm if a lord is executed.
Usurper armies that target specific factions for various reasons and may eventually form a kingdom.
Better unit collision (closer to warband's, but still able to move between your troops easily)
1-2-3 layers of walls and/or units surrounding the inner keep.
Improved effectiveness of spear walls against cavalry.
Disease that spreads, decreasing prosperity, and slowly draining troops from settlements.