I know sorcerer and also its subclass draconic bloodlines isn't in the game yet, but i know its planned to be. i wanted to make a special note on the fact that since the subclass only requires you to have an ancestry to a particular dragon type, that means they don't have to be in D&D 5e to be a valid choice, just means they had to be a official thing in any D&D edition. So even if your talking now extinct, that does not mean you don't still carry the linage. I think in all cases a dragon is immune to whatever damage its breath weapon is. With that in mind you can use dragons such as force dragon (3rd edition , did a cone shaped force breath attack), deep dragon (2e/3e, did a cone psychic attack.. which would be useful against mind flayers having resistance to psychic damage), and shadow dragons1e/2e/3e/5e, its breath was a necrotic cone. Just some information to help you do this right. when you make the subclass include in the sublcass a chance to have linage with at least one official dragon spanning from every edition of D&D to at least allow covering every spell damage type covered by all the games official dragon types. And again not just 5e. Consequently this is also true if you do the race dragonborn, you just need it to have lived at some point in any Edition of D&D to qualify.
Shadow dragons aren't a species per se. They are the result of a dragon getting pulled into and changed by the shadowfell. There's a shadow dragon in one of the campaign books who used to be a black dragon, for instance.
