So Airbender is out.

Initial pitch:

Former Fire aristos whose Clan was on the wrong side of the nationalist reforms of the Late Kyoshi Era settled in the North Pole as they transitioned to Merchants instead of nobles. Eventually, they intermarried with the Water Tribe and forgot their roots. But, every few generations, the gene pool throws out a Firebender. If Oa had been born a boy, the Elders would have known just what to do with her. But waterbending girls don't fight, and firebenders can't heal. She stowed away on a warship in male clothes and the men pretended to think she was a bit because "sweet Polly Oliver" was so darn useful, especially in the Arctic. Eventually though, she found an even higher calling, one that allowed her to be openly female. A diplomatic courier between the Northern Water Tribe and the Earth Kingdom. No Fire patrols would ever suspect a female firebender of such a role!
It so happens she was travelling from Rendezvous to Rendezvous when she heard of this...