It would be cool if there was a reinforcement system. For example, when you attack a small looter party that is being chased by a lord, I usually don't attack anymore since I know I won't be getting any loot or experience. It would be really good if there was a slight delay on when the AI lord can enter battle (since I engaged the looters first), and the delay would be anywhere from a few seconds to a minute or more, based on distance and party movement speed. Then the reinforcements enter from the red zone, total war style. Very immersive and improves gameplay a lot, and it leads to situations where you could be holding on until your reinforcements arrive, or you want to quickly eliminate the enemy so they don't double-up in numbers once their reinforcements arrive.
Also more tactical approaches for the AI can possibly be enforced just like the player can do - if reinforcements have long spawn time then maybe they'll be slightly quicker to attack you etc. A world of possibilities with this kind of system. If one party is defeated before reinforcements come, you can either continue the battle on the same map and in same position, or have a message pop-up and create another battle with adapted position. For example if I manage to conquer a castle before the enemy reinforcements come (huge army, slow movement speed & big distance), then I'll take the position of the defender in that battle - my troops on the walls defending from siege/enemy counterattack. Can keep the siege equipment where it was - siege tower on the wall, wall breached, gate breached, all of those minor details would be cool to see.