FF3 style Geomancer would be SO COOL. So definitely either that or a green mage because buffs are the shyz. Oh, or time mage. Thinking about having control over time without being bound by good game design just sounds broken and so incredibly cool. How does the world look when you use haste on yourself? The old spell is also scary as I'd have the power to make people age incredibly quickly. Not to mention having access to spells like slow, stop, float, regen, gravity, meteor and the amazing x-zone...
Alright then, definitely geomancer or time mage because good lord it sounds way too strong.