I don't like the great mage concept. I'm not sure how to include it exactly but for UU and Hero/King units of the civs we can tweak and tune it per unit (Sauron, Witch-King, Mouth of Sauron, Elrond, ...) and for the Istari we really have to see what is the best way to include them. Since we have Isengard as a civ, Saruman is already there anyway. For Gandalf, Radagast and the blue wizards, maybe they can be events or something or maybe a quest you have to fullfill to get one of them (and you can never complete more than one quest so you can never get two wizards for your civ). Also maybe restrict Gandalf to good, Radagast to neutral and the blue wizards to neutral and evil? Or something to that effect.