To allow certain wizards a bit more flexibility; similar to Cleric Path spontaneous casting, give specialists either a list of several spells from each school that they can burn spell slots to use freely, or open it up to the whole school.
This means a conjurer in dire straits could summon again instead of casting mords, or an abjurer could sacrifice other spell slots to keep counterspelling.
This would reflect mastery of that spell school and offer a bit more of a choice in spells for day-to-day without giving them infinite casts.