This is the problem, you can't really balance it. I would just give the player wings and thats it if it was my decision, like you get in aion, maybe even only for flying and not even fighting, hey they may be dragon knights, but learning to fly with a use of magic or something given from dragons will make more sense then tunning into a dragon out of the blue anyway. Going all dragon form is just a headache for the devs. I can only think of special levels that are meant to be played in dragon form that can attack ground units as a way to fix the unbalance that comes if they so chose to give u that option.
