I know this has been suggested before but bear with me.
New Location: The Flats (could be south of Alkir in some of the desert. This is where you unlock the skill line as the flats people are highly skilled in mounted combat.
New Weapon type: The lance/spear - a hybrid of one hand and shield and two handed. Can be used off the mount the lance might have a group effect where if used defence is increased (phalanx wall).
The Skill line: First skill would be to be able to use any one handed weapon (potentially a staff as well, could be a morph allows staff whereas the other morph would allow use of the bow).
Other skills would represent general mount roles e.g. Scout allows you travel faster on mount and is used to escape, Cataphract, makes you and your horse tanky so you can't fall of
the horse as easily and take more damage. Then you have shock cavalry which allows for a charge ability, having a lance equipped may give a bonus to this ability.