I'm not sure if this is where I should post this, so I'm hoping someone will redirect me if it isn't.

I was wondering about the stat basis for the bending, too. But especially in light of The Firebending Masters, wouldn't it make sense to base firebending on charisma?

After all, it is passion that gives fire its power in the Avatar world, whether that is rage (And a desire to destroy) or love (and a desire to protect).

Firebenders actually create flame, so I wouldn't consider it as much about wisdom (And an understanding of what is) as the other bending disciplines.
It is about the inner self and what can be created.

For Airbending I would argue for Intelligence, as it is all about finding clever ways to redirect the energy of the attacker- to disperse it.

Earth and water both seem more about wisdom to me. While Fire is offensive, and Air is Defensive, Water and Earth may choose easily what mode to be in and which is best for the occasion.

Thank you for your time. Happy gaming.