Experience is great already, but could be more awesome.
It'd be great if an army could (maybe after 1 turn of not moving) prepare some barricades or little fortifications, to make a piece of land more on the side of defending player, like it was that they made some camps, maybe had more strenght (becuase of eating and resting) and gathered some wood for temporarily defence. Even for more turns more wood could be avilable so more stuff army can build.
You could (to not make it so powerfull for the waiting or blocking army) do: 1 turn required to build a camp. AND after deciding to move again from camp an army could have reduced range of movement (again for 1 turn) to not be able to retreat so fast.
This would make fighting on land more strategic and creative, fronts could be a thing. Instead of making cities too hard to conquer and weaker armies so vulnerable. Fighting would be easy on land, medium on borders and middle-land and hard in cities.
Cheers, I LOVE YOUR GAMES.
(Poland not made avilable in Medieval campaign >:-] whyyy)