Split the Charred Skin in two parts. A sudden Fire Resistance of 20 is a bit too fast.
And add "Special: Able to cast Fireball" to the requisites.
Can you add others effects to your Fireball, like Spell Focus? I believe you can't add Metamagic feats, seeing as it doesn't have a "higher level spell" to use. Can you use your normal memorized Fireballs spells with the effects from your ability, like casting a normal Fireball and it getting the benefit of Empower the Fireball or Maximize the Fireball?