I think since aang destiny has never been defeat Ozai, but end the war and restore balance, Ozai will be deadthroned by Azula, so Aang's mission will be defeat her! Ozai, will help her, since he'll think "wtf! I'll help her anyway, she's my daughter and is so intelligent... She'll win the war and it's not convenient for me to go againgst her, but Zuko will kill/defeat him, that wouldn't affect Aang's destiny at all. After Ozai's deadthrone, there will be a civil war, ended by Aang and Zuko will be the new FL, he'll end all problem in his nation, untill.... (see fourth season: air.)
(Azula will try to deadthrone Zuko (about 4 years later) she'll secretly will create another civil war), until (ang's dead, the Avatar is Katara and Zuko's younger daughter, a waterbender, like Katara, she'll have an older waterbender or nonbender brother, but the eldest sister is a firebender like Zuko, she'll be the heir to throne, so Azula thougth :"with such amount of kids I need a civil war to controll this west southern empire!, until the airbenders arive to help Zukoand Katara...)