How are we feeling about 5.5e Heroes of Faerûn's circle magic as a direct, free expansion to spellcaster flexibility and problem-solving, right from level 1?
https://5e.tools/book.html#frhof,5,circle%20magic
Set aside theorycrafting-level options like hours-long Delayed Blast Fireballs for gajillions of damage, or outdated wording of legacy spells like Earth Tremor. Consider some more practical applications.
A level 1 party wants to assassinate some pesky noble from afar. A longbow goes out to only 600 feet, and even that will probably incur disadvantage. No problem: just circle cast Chromatic Orb, Guiding Bolt, Witch Bolt, etc. for +1,000 feet of range per secondary caster. The secondary casters need not expend any spell slots for this.
A level 1 party wants to Sleep the enemy side, but a 5-foot radius is insufficient. A 15-foot radius, though, that might do. A secondary caster tosses in their own action and spell slot, and there we go.
A level 3 party wants to squeeze more out of Detect Thoughts. They get one secondary caster to burn a level 1 (yes, just level 1) spell slot for +1 hour of duration, bringing such a spell up from 1 minutes to 61 minutes.
These are merely level 1 and 2 spells. Who knows what a higher-level party can do?