I think it would be a good idea to come out with spells that require a specific combination of schools to be able to train. For example an ice wizard who has trained all the life spells would unlock a spell that is both ice and life. This would give more incentive to fully train a secondary school instead of just a little of each. Such spells would also need to allow for power pips to be used from either school mastery to cast.
The idea is okay, but some school combinations are better and more popular than others. It would be kinda unfair since some school combinations aren't very useful and some wizards don't even want a secondary school. Many do though, so I don't know why this "incentive" is necessary.
