Just got my copy. May I say that my wife and I really love this game, and we would definitely want to see more expansions in the future.
As for spells I would love to see...
1). An absolute single creature removal card- After having a test turtle-ing battle with my wife, there came a time when the board was just filled with creatures (and having spells under them increased the clutter). Neither mages want to move closer because some creatures have sweeping attacks in addition to the volume of creatures in the center zone. I think having an absolute creature removal card would be nice. Making it a level 4 card would also help, so players wont be able to spam them (provided that it cant be attached to any wand). I believe that it would quicken the pace per game. As for a spell name.... Execution?
2). A global removal card- This concept would be interesting for solo builds. Allowing players to wipe the board completely of any object except for the mages. A last resort if you will. I'm thinking... a level 6 legendary epic fire spell... Apocalypse?
3). (this may be a long shot but.... here goes) Blink/Flash Step- you can only use your quickcast token to use this card. Move your mage 1 zone away. (this would be 0 to cast).
-Since I've seen a lot of turtle strategies, allowing attackers to maneuver and cast a full action spell would be nice. Once again, I believe that cards like this would make bouts quicker.