I don't think Azula could redeem herself. If she ever joins the Aang side, it'll be because it's convenient for her, and I wouldn't be surprised if she double-crosses them had she got a chance. Personally, I think she's cooler as a baddie, I hope she stays as such (even if she'll eventually lose because it's a children cartoon after all), and a redemption for her would be kind of lame considering how she is.

I'm hoping we get to meet exiled Ursa :3