A good idea would be making garrisons develop independently of player and lords and an ability to leave army in a fiefdom, so they aren't garrisoned but only stationing in the fief or if you haven't got your fief nearby and you are on a war campaign, an ability to make a camp and leave your heavy and slow troops here and take some light horsemen and scout the area for enemies. Also it would be cool if they implemented the AD1257 Lance Recruitment or some sort of taking levies from your lands, because medieval lords didn't personally train their troops and maybe make the upgrading of troops more dependent on money than on gained experience, for example you have 30k denars and your army just got slaughtered by enemies and you have to rebuild it, then you spend your money on better soldiers.
Another good idea would be to make soldiers work like in MTW2, so you don't "upgrade" them into better types, (e.g man at arms to knight) but every "level up" gives them higher level in combat, for example a rank 1 man at arms has got level 10, rank 2 level 15, rank 3, level 20 etc. and equipment levels (e.g knight with basic weapons have a cheap lance and a light sword and with upgraded weapons, a great lance and a heavy sword etc).