On the topic at-hand, fire is the most fleshed out of any elemental damage type and it's also the most resisted besides... I think... poison. Poison is sort of a special snowflake and there are just a ton of different categories of creatures that are straight-up immune to it.
Scorching Ray in particular is also interesting, as written I believe, in that it gains the benefit of the dragon sorcerer's damage multiple times.
Another note is that in this edition, unlike 3e/3.5e/Pathfinder at least, you don't need a ton of spells to cover your damage needs. You just need some general bases covered as a sorcerer. You need something good for single target, something good as a single-action cantrip, and something for AOE and/or Multi-target.
Acid has a cantrip, and it's actually one of the higher damage cantrips when 2 enemies are next to each other, with the disadvantage being that the damage is spread out over multiple targets (which is kinda fine for dragon sorcerer since you'll add +Cha to damage on both as well).
Chromatic Orb isn't... bad... but it's not competitive with most things.
Melf's Acid Arrow doesn't seem that bad to me. You're also adding +Cha damage on the second turn, and you can twin-spell it. It's fairly par with scorching ray till you start stacking up those rays and those extra +Cha modifiers.
What's really missing is an AOE or multi-target effect. Acid Fog is an iconic one from previous editions that you might be able to re-skin.
http://www.d20srd.org/srd/spells/acidFog.htm
http://www.d20pfsrd.com/magic/all-spells/a/acid-fog
6th level spells though are pretty high for your first real aoe damage. Either way, at my table I'd try to figure out something reasonable to make any element work. I also want them to all have slightly different special things about them, and fire seems to be straight up about burning things with damage. That seems like the gimmick it has been given. Acid... so-far (ignoring Chromatic Sphere) seems to be multi-hit (either multiple targets or multiple rounds) where the damage might even be a little bit better than fire if whatever special circumstances are met and you don't mind the fact your damage is spread out (over multiple targets or multiple turns).