It strikes me that the mod community will be able to add new spells to the game (probably?).
I'd very much like to see spells added in thematic groups, that way you could choose one or more groups of spells to use for your character as they progress in the game. So I would start off the game with the option of choosing the spell book of Ranulf of Flambeau, Hermanus the Architect, or Lambert the Trader.