Ok, so I'm well aware this is an old topic, but I just had some new ideas. Hear me out.*
Katara- Victoria Justice (I really don't like her very much, but I think she looks like Katara and could pull off her nurturing persona) with blue colored contacts, or Taylor Dooley ( who is a really fantastic choice because she is younger looking and could be a better couple with young Noah).
Sokka- Avan Jogia with blue colored contacts. (I also think Luke Benward would be a good choice, but he wouldn't correspond with the darker-skinned Victoria/Taylor).
Aang- I actually think Noah Ringer was aptly cast, it was the script with the problem.
Toph- I personally like Isabelle Fuhrman, but since much of the fanbase is gearing towards an Asian actress, I would say an unknown, or perhaps that girl from Karate Kid.
Zuko- JUSTIN BIEBER!! Just kidding. In actuality, this one has got me totally stumped. I'd probably say another unknown Asian actor.
Jet- Drew Roy
Smellerbee- Chloe Moretz
Longshot- Josh Peck.
Yue- Same actress.
Azula- Kelly Vitz
Ty Lee- Ashley Argota
Mai- Christian Serratos (sp?)
Ozai- Jet Li
Suki- Alexandra Daddario
Ursa- Lucy Liu
Iroh- ? Not too familiar with Asian actors/actresses, sadly...
So that's my cast. Look them up, and tell me what you think.
- I do not take into account the physical abilities of any candidate.