Feat in question is called fiery burst (see bellow). Problem: prerequisite is 2nd level spell available. That can be solved by taking Precicous Apprentice. But then I loose Draconic Heritage since I need TWO feats to make it work. This combination should help with whole day casting vs. orcs and goblins and is very powerful on 1st through 3rd levels - later it simply gives one more 2nd level spell to cast.
Those are feats from complete Arcane except for draconic vigor which is from dragon magic. IF you don't like draconic vigor then I'll take draconic resistance from CArcana
As for heritage feats, there is not much to it. It simply allows sorcerer to get bonus feat from heritage feats at times when wizards gets metamagic bonus feats (I believe every 5 levels?)
So, there are two versions, I'll take whichever you feel more comfortable with:
1. Feats: Precocius Apprentice, Fiery Burst
[sblock=Precocius Apprentice]
Your master has shown you the basics of a spell beyond the normal limits of your experience and training.
Prerequisite: Spellcasting ability (Int or Cha) 15, arcane caster level 1st.
Benefit: Choose one 2nd-level spell from a school of magic you have access to. You gain an extra 2nd-level spell slot that must be used initially to cast only the chosen spell. Until your level is high enough to allow you to cast 2nd-level spells, you must succeed on a DC 8 caster level check to successfully cast this spell; if you fail, the spell is miscast to no effect. Your caster level with the chosen spell is your normal caster level, even if this level is insufficient to cast the spell under normal circumstances.
When you become able to cast 2nd-level spells, you lose the benefit described above but retain the extra 2nd-level spell slot, which you can use to prepare or spontaneously cast a spell of 2nd level or lower as you normally would.
Finally, you gain a +2 bonus on all Spellcraft checks.
Special: You can take this feat only as a 1st level character.
[/sblock]
[sblock=Fiery Burst]
Fiery Burst
Type: Reserve
Source: Complete Mage
You channel your magical talent into a blast of fire.
Prerequisite: Ability to cast 2nd-level spells.
Benefit: As long as you have a fire spell of 2nd level or higher available to cast, you can spend a standard action to create a 5-foot-radius burst of fire at a range of 30 feet. This burst deals 1d6 points of fire damage per level of the highest-level fire spell you have available to cast. A successful Reflex save halves the damage.
As a secondary benefit, you gain a +1 competence bonus to your caster level when casting fire spells.
[/sblock]
2. Feats: Draconic Heritage, Draconic Vigor (this one is from dragon magic, I understand if you disallow it)
[sblock=Draconic Vigor]
You gain some of the vitality of your draconic ancestry when casting spells.
Prerequisite: Draconic Heritage, sorcerer level 1st.
Benefit: Whenever you cast an arcane spell, you heal a number of points of damage equal to the spell's level.
[/sblock]
I dislike breath weapon as it takes spells out of use...