This premise is so great and I am seriously intrigued. What could a threat possibly be that they'd need Azula to help? Sure she brings a lot of power but you'd have to unshackle her to get it; she also could contribute a whole lot to strategy & tactics if you could trust her not to play you false. But Toph's comment about Aang wanting to give her another chance catches my attention; if there were alternatives to Azula's help then Aang probably leaned toward Azula because he is a silly thing like that...

ANYWAY I seriously liked this and it got me thinking, so! Awesome.