I'd like to create a mage that specializes in using the natural terrain to his advantage, enhancing it in various fashions to the tactical benefit of the unit, rather than some fireball slinger.

If forced to pick a specific single ability, I'd go with either rapid plant growth or the summoning of a fog. If you give me a better sense of the scope of initial abilities, I'll be happy to conform to that standard. ;)

Barring physical alteration of the landscape, I'll happily play an illusionist. I can go several ways on this, but the mage I envision is almost an avatar of "By way of Deception, thou shalt wage war." So perhaps an affinity for shadow is pertinent here.