Transformation - Morphs the player to look like the target of the spell. This would let Mages do what Rogues do but in a completely different way.
Want to get past that guard at the gate ?
Rogue: Sneak
Mage: Morph into a guard and walk past
Want to convince that quest NPC to give up cricital information ?
Rogue: Diplomacy
Mage: Morph into his friend and ask him
Want to open that locked chest in the backroom of the inn ?
Rogue: Picklock
Mage: Morph into the owner of the inn and ask for the key
This would open up ALOT of different options on how to play the game, and how to influence the world around you.