I admit, the idea of a mage tank I love. I feel it is not something I have ever really seen done as its usually the tin can variety. I do admit however when I hear shapeshifter, I think more along the lines of say Terra and Espers. The gauge being a bit different in that it starts at 100 and while in Esper form it counts down, quickly recharging out of combat but also able to be built up while in human form.