The first idea to jump out at me is a Dream Mage.  Besides the obvious sleep inflicting effects I would imagine that there would be spells that take advantage of this.  A spell that allows the Dream Mage to take control of any sleeping creature or that regained life or mana per each creature on the field that is alseep.  They could have their own brand of creatures called Phantasms or some such that have advantages against sleeping creatures or are able to induce sleep.  A dream travel spell that lets the caster appear next to any sleeping creature.  Maybe a spell that let the mage escape to the land of dreams temporarily (i.e. leave the board for a turn).

Another idea would be like a technomancer.  He would focus around creating items and constructs/automotons.  He could create automatons with modular accessories represented by item cards that would be equipped to them.  This would allow the Technomancer to adapt his creations to his needs and allows him to adjust his forces on the fly.  His versatility would be counter-balanced by a lack of direct firepower.

Another one I think would be interesting would be a shapechanger.  It would have the innate ability to pay X amount of mana to replace his character with creature cards from your spellbook.  The strength of the creature would help determine cost and there would be an upkeep cost to maintain the effect.  Maybe, as a balance, the mage cannot regain mana while in a different shape?  Another possibility is that he could change out a creature he has on the field by paying the difference in mana or thereabouts.

Just a few suggestions.

I just went ahead and picked up a second copy of the main game but I'm surprised they don't offer the board as a standalone product as I've seen some other companies do.  A fifteenish dollar pricepoint would be good I think.