It'd be nice to see the ol' Mirror Magic get an overhaul. As it's described during the convocation, it's basically a "Duelist's Stance" for spells. It'd be more interesting if it was that sort of thing, nullifying an enemy spell and then immediately using it as a free action. From a more mechanical perspective, Mirror Stance being a stance makes it basically a nonstarter for magic-focused builds, which can usually afford to give up an at-will, but have extremely compelling encounter slot "stances". To say nothing of it being a tank-oriented ability - without the defensive utility this usually entails.