Interesting idea. Although I don't agree with a lot of the OP, it is an interesting thought experiment. What about the Ethereal-Color combos (which, I know isn't really a thing in M:tG, but it is still an interesting concept to me)?
Ethereal-Divine (Colorless-White): Arcana Clerics? Banishment? Aura abilities (like Paladins' Aura of Protection, etc)? Maybe Astral Transcendence (like Monks' Astral Projection/Way of the Astral Self)?
Ethereal-Psionic (Colorless-Blue): Psychokinesis, Mind-Melting, creating matter from mind/thought.
Ethereal-Shadow (Colorless-Black): Shadow-Travel, Phantom Magic, creating objects from shadow-stuff (Shadow-Blade, Illusory Dragon), becoming incorporeal?
Ethereal-Elemental (Colorless-Red): Elemental Chaos? Wild Magic Sorcerers? Genasi?
Ethereal-Primal (Colorless-Green): Fey?