It is great the developers managed to implement at least simplified horseback combat to the game, including horseback archery just before the release.
I think improving polearms like spears/halberds and horseback melee combat with polearms (to the level of something like M&B Warband) would be the greatest improvement of combat mechanics in the game.
Polearms and horseback combat were 70% of medieval combat
Fighting on food with backup weapon like mace, sword or axe is great also but it was about 30% of medieval combat, so just improve polearms and horseback a little.
BTW. The game is just great