Yaarel
🇮🇱 🇺🇦 He-Mage
This is a great idea, make the Sorcerer a planar mage.I'd really like to see sorcerers have powers based on planar origin. Dragons etc. are Material Origin, Elemental Origin, Outer Planar Origin, Fey, and Shadow, and then sub-options for each subclass there based on the specifics.
It covers themes that the 5e Sorcerer already includes, and pulls them altoghether in an overall concept that is interesting.
Structurally, maybe organize the class options as something like plane and subplane.
Material: draconic, or giant
Ethereal: fey, shadow, or ether
Elemental: earth, fire, air, water, or elemental chaos
Astral: celestial, fiend, or aster
So for example, the Chaos Sorcerer is specifically an elemental Sorcerer who specifically attunes the plane of Elemental Chaos.
Moreover, the Sorcerer should be able to planeshift back and forth between the current plane and the innately sorcerous plane. There should also be a special affinity between the Sorcerer and the inhabitants of the innate plane.
This connection between the Sorcerer and these otherworldly spirits, like members of the same family, helps make better sense of what the term "sorcerer" means. It is the employment of a spirit to perform a magical effect, especially a harmful one. Here the D&D Sorcerer does the magical effect oneself innately, but to have a special affinity with these other spirits feels correct.