Just thought I would throw this suggestion out. What if a few more main schools of magic were created this way wizards who already have multiple wizards would find a refreshing new way to play the whole game again with new wizards? I myself have almost two or three wizards of each school and find the repetition of it getting a tad stale let me know your thoughts. This would also keep some folks entertained longer and give you more time on developing new worlds.
I don't think so, I'm fine with side schools, but new main schools aren't a good idea. You would have to create new spells AND quests for it, and you need to figure out where to put it's school, I mean the physical building. And they most likely would require an opposite school for them, new teachers, enemies and bosses for each new school. Clothing, a house themed for the new school, and all that while still coming out with other upgrades.
Side schools I think are fine because we add those as we go along.