My two suggestions are...Potions or buffs you can give your general in an adventure or elsewhere which would enhance various of his abilities for some period of time. Perhaps the ability to earn or purchase such potions or battle buffs could be a leveling perk.
Second idea is the ability to earn or unlock increasingly fast steeds for your generals. Whether such faster steeds would be achieves, level perks or purchases could be decided later, but the idea is that as you progress it becomes possible to acquire ever faster steeds.