I've decided to re-role the characters for the movie, this combination should do well

Zuko: Keanu Reeves
I believe Keanu Reeves earned this role with the line in "The Day the Earth Stood Still," (Regina: this is our planet) "No... It is not."

Aang: Gerard Butler
Gerard Butler (aka: King Leonidus) is the perfect actor for Aang, because, even though it would change his character completely, Aang would be replaced by... this guy: http://www.youtube.com/watch?v=rZBA0SKmQy8 (THIS IS SPARTA!)

Katara: Angelina Jolie
Angelina Jolie earned this role by posing for the movie poster of Beowulf, enough said.

Ozai or King Boomy or several others: Lewis Black
When you need an off-the-hook character you need a comedian, and Lewis Black's voice and attitude are perfect.

Iroh: Jackie Chan
Lawl

Sokka: Matt Damon
Matt Damon earned this role through the endless scenes of Jason Bourne beating enemies senceless in 3 seperate films.

Jet: Bradd Pit
yes... who else?

Avatar Roku: Morgan Freeman
"Now let me get this straight. You think, that your client, one of the wealthiest most powerful men in the world, is secretly a masked vigilantee who beats criminals to a pulp in the night with his bear hands, and you intend to blackmail this person?"

Fire Nation Admiral (forgot name): Admiral Ackbar
This role can't repel acting power of that magnitude!