I've been thinking on Cooperative Magic in order to get that cinematic feeling of the frontliners defending the casters in clutch situations and big scale magic retinues on a battlefield. It could replace spell scrolls entirely with formulas for this casting in harder settings.
Cooperative Magic allows a group of spellcasters to cast a spell they couldn’t otherwise, and to combine their talents and resources. To cast the spell, they must contribute spell slots to build up to the power of the target spell slot. Two spell slots of level n combine to produce level n+1 (so four nth-level spell slots to produce n+2, etc). Contributing a spell slot requires an action and concentration. If a spellcaster’s concentration is broken before the combined spell is cast, the spell slot they contributed is lost, and also the rest of the spellcasters take a penalty to concentration. Once enough spell slots have been contributed, casting the spell requires another action (and concentration has to be maintained until then) by one of the members of the group. The final, combined spell is not only higher-level than any of the spell slots they used, but also benefits from all of their feats, features, and metamagic.
Really a situational casting, but it's there; forces at least the expenditure of a whole turn to feed the slots during combat, and it gets those tense scenes. It allows low lvel casters to be useful in big casters spells. It allows low level casters to pool their resources and cast higher-than-known spell scrolls they find. Also allows casting 7+ spells more than once a day.
How balanced/overpowered would that be?