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.
Your suggestion isn't bad but consider the following. If they make this concession to port material from previous editions for this then where do they draw the line without upsetting everyone else?
The line is if it is official and if its referencing the past. Previous versions of D&D( ie 4e and older) are valid as history or bloodlines in 5e. I am not saying they bring something back from the dead like bring back force dragons, but the subclass draconic bloodlines and he race dragonborn do ask the player where their ancestry is rooted. In the case of dragonborn race and the sorcerer subclass using a force dragon as the ancestry is legal and almost intended by the wording, because they left the wording open so you didn't have to be chained to a few choices, the chart given is listed as examples, but isn't the definitive list of choices to choose from. No where written does it say you had to choose from that list, or only 5e content, and looking at how its framed, totally out of the spirit of the question to assume only 5e content would be valid. That is what makes it perfectly acceptable to push past 5e into other editions, when there asking where or what in time you came from, that is the line.
