The fact remains that, outside of fire and possibly ice, there are few options for the sorcerer to use his bonus damage ability. When making a selection for dragon bloodlines, there is an overwhelmingly best choice (fire), a decent choice (ice), a poor choice (lightening) and two trap choices (acid, poison). Even adding a supplement doesn't change this arithmetic. While I'll be the first to say that 5e isn't much about balance, I at least expect there to not be trap choices lurking in the sorcerer class.
And my second point is that this is easily corrected by the addition of a relatively small selection of spells -- ones targeted to address this lack. Or the inclusion of a special metamagic, specific to the bloodline, that allows for the substitution of energies to the favored energy of the bloodline only. Done. Fixed. No issues, no major power gained.
There is a cantrip for each type that scales with level, and will always be available for them. When it comes to actual spells beyond cantrips, fire does indeed dominate, but unless you think sorcs are supposed to be locked out of other elements or something, it shouldn't matter. The bulk of your cha mod to damage will be coming from your cantrips.
Anyway, I wouldn't consider either acid or poison to be "trap" choices. Acid Splash and Poison Spray are both pretty good spells under common circumstances. Both bypass armor, relying instead on saving throws, which is often kind of useful. Acid Splash is more geared as a mini-aoe, and Poison Spray is single target, but otherwise both are good. The only real argument I can see being made against either is that poison resistance and immunity is fairly common with certain types of creatures, so if you're campaign is going to be dealing with a something like an undead theme the whole way through, then perhaps it's not so great. Even then, it's not like the sorc won't have steady access to other spells and cantrips to deal with that when it comes up (fire is common, as you point out).
Unless you're just trying to min/max everything and coming to the conclusion that anything that's not the best isn't worth taking, then I'm just not really concerned.