I personally would love to see the current mage class split into evoker and summoner and then another support or close-range type mage added.
There's an app for that!
This game really could use more spells and Demons and Wizards definitely fills the need: it introduces two whole new schools of magic and creates a summoning pool that makes being a summoner interesting to play, and gives a regular mage better options as well.
Another problem with magic currently is the lame "one time" use spells. Only one hero or sov can have tireless march?!? Only one can be annointed by fire? And the vanilla summoning system is awful!
Couldn't there be a trait that makes it possible to cast such spells multiple times? And if you have the trait you could still increase the second and tertiary casting cost/upkeep of the spell if you were concerned about spell spamming.
The DLC spells could all be unlocked by quests, giving additional material for the game, as well.