I am thinking of a 'combine' spell. There could be two versions of this spell.
The 'lite' version would allow the wizard to apply a second boost to their spell cards. Balance wizards would be able to put accuracy and monstrology on one card.
The 'heavy' version creates a treasure card from two spell cards. This would allow Balance wizards to cast two spells at the same time.
As balance combines elements from all other schools, this seems to be a natural progression.