Building on an earlier post, I would potentially consider a setup like this for the full-casters:
- Arcanist (arcane/order): astral, mana, time, gravity, sigils, etc.
- Warlock (shadow): necromancy, shadow, void, dark arts, blood, etc.
- Mystic (mental): mind magic, illusions, enchantment, psionics, etc.
- Druid (nature): animals, plants, weather, land, etc.
- Elementalist (primal/chaos): air, earth, fire, water, etc.
- Cleric (divine): holy, healing, buffing, abjuration, etc.
I would also consider just flat-out giving the players the ability to choose their primary ability casting score for the class, possibly from a list of two out of the three choices: e.g., Cleric (Wisdom or Charisma), Mystic (Intelligence or Wisdom), Warlock (Intelligence or Charisma), etc. However, once you pick it, the casting attribute stays the same for that class.