The movie shouldn't star with aang's origins or with the beggining of the war or when the spirit was sealed (by the way, I think we shouldn't use Avatar Roku in here, we need an older one, like Kyoshi or even an older one).
The moovie should start with a scene with one of the final fights, then, the movie should continue showing the public what happened to end like that. EXAMPLE:
(This is the movie script order, how would look like in the teathers, not the order we should create it)
1?? Iroh vs Gang and fire siblings (if you want to we could put some kyoshi warriors and freedom fighter)
2?? (past) Ozai being defeated
3?? (after Ozai's defeat) Iroh going mad with his power, and fighting against Azula.
4?? Iroh vs Gang and fire siblings (if you want to we could put some kyoshi warriors and freedom fighter) (how continues the fight)
5?? (past) How a spirit the public doesn't know anything about is sealed by Koh, and an Avatar
6?? (after 5??) How started the war.
7?? ( Iroh vs gang Fight continues) Aang's high air bending skills/level.
8?? (past) Aang's origins and life at the air temple.
9?? Like 7?? but with Water siblings
10?? Just as 8 but with Fire siblings
11?? Some Ursa and Ozai relationship.
12?? How the gaang joined
13?? then we put all the rest of the movie with a chronological order, and we managed to make people understand everything when there are just 30 minutes left to the end of the movie.