Incenjucar
Legend
You can combine a LOT of these into a build-your-own. Most monsters in 5E with any complexity are casters, and you can build that into subclasses. A lot of things have similar features, too. Outsiders like fiends and celestials mostly have special sight and strong defenses, a medusa and a basilisk barely differ, a siren and a harpy vary mainly by terrain, etc. The harder parts are the magic weapon gap and the stat array.Ironically, the closest we got to this was the playtest sorcerer, which transformed as it spent its will points.
But yeah, this one is basically non existent in DnD, and could easily be its own class. Lots of monster variants like vampires, werewolves, liches, and all sorts where players could benefit from being able to play as a version of them.