Void Mage.
Come on, it totally fits, and the next expansion can add ethereals so it fits even more. We have done the elements as casters, arcane, fire, frost. Locks have shadow and fire damage and summons, shamans have elemental damage, priests and pallys holy damage...
What's left other than void magic? They area already teasing it in Argus zones, it makes perfect sense for the first ever caster hero class.