There are five things I would absolutely love to see in this game:
1. Cultural Abilities - Basically, each faction would have unique abilities available to it based on the cultures they were inspired by. As an example, as the Imperials you have the option to build roads, aqueducts, and other such infrastructures. Alternatively as Nordic forces you have the option to loot and pillage castles and towns you successfully siege which will garner huge amount of money and goods, but but will reduce the castle/towns prosperity to the minimum and slow its subsequent recovery to a degree for a few months.
2. Expanded Troop Trees - If any of you have played the Floris Mod you know what I mean.
3. Tactics - Specifically, it would be nice to see your options in regards to troop command shift with levels into the skills. For example, at level 0 you only have the option to tell men to group up or go somewhere; but as you get more levels you get the options to tell your troops to form ranks or shield walls, to shield-lock, to have archers volley fire, to have your horse archers circle around enemy infantry, etc.
4. Divisions - I would love to have the ability to have divisions and sort which troops get placed into these divisions. Furthermore, if the add a preplanning stage to battles, like in the Floris Mod, then you could choose how these divisions initially spawn on the battlefield. Also have the ability to raise soldiers (or install companions) as captains over those divisions who would make those soldiers movements more efficient and cause them to fight better, but with the obvious downside that if the captain is killed or incapacitated the division may falter or even break lines.
5. Realism Mode - The game just become more realistic (and likely more difficult), for example you now have to carry water with you as well as food. You now require pack animals (this can include horses used by party members) to carry large amounts of goods.