On the downside, my brain is still a bit fried, so I'm going to throw this post out there and not worry about whether it comes to a logical conclusion or not. Just some ideas from years ago.
If you want to see me froth and foam, tell me how logical and well-balanced spell schools are in D&D. This was fairly irrelevant in 1st Edition AD&D, but with the advent of specialists in 2E, school mattered. The problem in part was a lack of consistency between schools about cause and effect. An evocation "creates" energy, but an illusion "creates" an image, which may or may not have a material effect. Unless it's a shadow illusion, in which case it "brings" shadow energy, which would normally make it a conjuration. Abjurations do whatever they want, as long as it keeps things away.
Back when I was hacking on 3e, I tried two different ways of taming spells & their schools: descriptors and sub-schools. The descriptors option wasn't bad, but I really sort of like the sub-schools, in part because the foundation was already laid in illusions, conjurations and haphazardly in a few other schools. I'm not sure why it wasn't ever extended to the rest of the schools: sacred cow or historical ennui, I suppose.
So, here's a breakdown of 3e spell schools and sub-schools.
Abjuration: Barrier, Condemnation (ie banishment), Negation, Protection (a person or creature, usually mobile), Warding (an item or location, usually static)
Conjuration: Calling, Creation, Healing, Summoning, Teleportation
Divination: Awareness (detect something), Lore (know something), Precognition (foretell something), Psychic (read your mind, which could also be enchantment), Scrying (see something)
Enchantment: Charm (make them want it), Compulsion (make them do it), Morale (make them feel good about it).
Evocation: Elemental, Energy, Force, Positive Energy, Negative Energy.
Illusion: Figment, Glamer, Pattern, Phantasm, Shadow
Necromancy: Curses (could move to Enchantment as well), Death, Flesh, Life, Spirit, Undeath
Transmutation: Alchemical (non-living target), Enhancement (buff), Manipulation, Transformation (living target)
This has been on my mind while I've been working through spells for illusionists and variant illusionists. I won't put schools or subschools in the spell descriptions (has no meaning in S&W), but I will list them in an appendix. The biggest change will be adding a Radiant subschool to Illusion, which absorbs the prismatic, chromatic, iridescent, and color spells (and maybe the patterns). I like illusionists as masters of light as well as shadow.
Schools also seemed like an unexplored area for customization. With a few exceptions (elemental schools, wild magic, and chronomancy), the idea of new schools was never developed. Why shouldn't there be a School of Elven Magic? Enjoy.
0 - arcane mark, detect magic, light
1 - arrow mind1, charm person, dawnburst3, dispel ward1, endure elements, entice gift1, ghost touch armor1, guided shot1, guiding light1, lesser deflect2, light of Lunia1, mage armor, magic weapon, spontaneous search1, surefooted stride1
2 - deflect2, eagle's splendor, protection from arrows, see invisibility, stay the hand2, sure strike2
3 - arcane sight, crown of clarity2, crown of protection2, daylight, dispel magic, ghost lantern3, glowing orb1, greater mage armor1, greater magic weapon, heroism, light of Venya1, mask of the ideal3, spellcaster's bane3
4 - charm monster, detect scrying, lesser geas, Melf's slumber arrows3, spell enhancer1
5 - break enchantment, dominate person, dragonsight1, lucent lance1, Mordenkainen's faithful hound, radiance2, sending
6 - brilliant blade1, geas, greater dispel magic, greater heroism, Tenser's transformation, true seeing
7 - greater arcane sight, spell turning, synostodweomer1
8 - mass charm monster, mind blank, protection from spells, sunburst
9 - genius loci3, Mordenkainen's disjunction, unbinding1
1 Spell Compendium
2 Player’s Handbook 2
3 Complete Mage