Also, Bumi definitely moves through earth like it's water after his fight with Aang.
Also also, a feat that was probably forgotten on the first thread:
prerequisites:Con 12, 4 ranks in knowledge (bending), Firebender 1st level
Your breath control allows you to add your constitution modifier to damage rolls for any damage dealing fire seed.
Too much for a first level feat? I think it would be balanced out by the choice between increasing constitution for more damage or increasing wisdom for better bending checks, and would be good for Zuko-like multiclassing. He's clearly got a punch behind his fire, but doesn't know quite so many seeds and forms as a more optimized firebender like his uncle or sister.