"Reducing the classes to their basic cores (Warrior, Mage, Cleric, Rogue,...)," particularly the Fab Four, is not quite what I had in mind. It's a difference of em-phah-sis so to speak. The core of the spellcaster would be based on their casting style rather than whether they are a Mage or Cleric.
So you mean a learned caster (wizard), an innate caster (sorcerer), a patron caster (warlock), and a deity granted caster (cleric). Then have base class features and casting stat based on the method. Then finally have your bloodlines or domains or whatever choose your power source and associated spell list.
So a draconic sorcerer, spellschool wizard, GOO warlock, and knowledge cleric uses an arcane list.
The divine sorcerer, celestial warlock, and light cleric uses the divine list.
The storm sorcerer, fey warlock, tempest cleric, and the shaman wizard uses the primal spell list.
etc etc