I think you got the immunity from the Pathfinder 3.5 rule setting and 3.0 and 3.5 D&D edition rules on the elemental breath weapon attack. In those editions you gain total immunity to your associated element by the end of the Sorcerer bloodline or Dragon Disciple prestige class. I have no idea how it is done in 5E so just explaining how there might have been a misconception to being immunity to your associated element for the build. As far as Force Dragons and Shadow Dragons...I think the best you are going to get is Metallic or Chromatic dragon ancestry since they are the prevalent types in most of the multi-verse and Faerun.
I mean technically there are Prismatic Dragons lol. Rainbow Breath attack! TASTE THE RAINBOW! :p
Kidding aside, even if the 5E gives you a sizable resistance to your element type you choose still better than nothing. Also saves on having to cast Resist Elements or allows you to pick another element to be resistant too! Oh, the possibilities for magic! I can't wait to be a Sorcerer! Whee! Wizards beware my mighty innate power! Huzzah! *Blinks* Um, btw, you got a scroll of mage armor I can have forgot to innately have that spell from my bloodline!
Okay I lied, I can't stop joking about. However, I am psyched to see Sorcerer. It is my most favorite magic class I never got or get to play except in Baldur's Gate and NWN games due to someone was always a sorcerer at my gaming table. I am sure though when it is released it will be awesome!
