Master of the Elements (passive) - Each time you score a hit with a single-target Arcane, Frost, or Fire (mage) spell, you gain a buff which increases the damage of the next spell of a different school by 10%, stacks up to 10 times. (e.g. if you cast Fireball, you gain 1 stack of 10% Arcane Bonus and 1 stack of 10% Frost Bonus).
This would really mix up the typical mage rotation by introducing spells from other schools as occasionally very strong. Like Fire casting FFB as a super-nuke. We would need a spell that's an arcane/frost or arcane/fire hybrid since there's no Arcane spells which are accessible as Fire/Frost atm.