If we make a Dark/Mind creature, the cost per mage would be very interesting. Assuming Level 1 Dark and Level 1 Mind, we'd end up with:
Priestess/Priest: 5
Warlock/Warlock/Necro: 3
Wizard: 4
Beastmaster/Beastmaster: 4
Forcemaster: 2
Warlord/Warlord: 4
Druid: 4
This is because the way the Forcemaster is worded, if the creature has any bit of the Mind school in it at all then it is considered a Mind Creature and the Forcemaster gets it at base cost. So no matter how we split the schools, the Forcemaster will get it cheaper than anyone else. Level 3 Dark and Level 1 Mind? Forcemaster pays 4 and Dark Mages pay 5. Kind of cool.
Anyways, my vote is indeed for a creature. I think that would be the most fun to make.