Iroh will die as part of Zuko's hero's journey but Iroh could still start teaching Aang before he dies.
Azula will probably die. One big enemy per Book. (1: Zhou, 2: Azula, 3: Ozai)
I honestly don't care about the relationships but Aang is going to end up with Katara, it's kinda obvious... I don't get whats with all these weird relationships to be honest...
Will Aang master the elements in time? No, he won't master them in time and the whole world will be taken over by the Fire Nation then everyone will die. [/sarcasm]
Will Aang master a sub-element like lightning or plants? Maybe, probably not until Book 4, if there is one, then he'll master all those sub-elements.
Will we see Ozai's face? Yea
What do you think of Zuko or Iroh becoming Fire Lord if Ozai dies? Yea but they wouldn't call them the Fire Lord, something less dictatory
Will Aang die in battle? Nope.
Book 4? Yea. It will probably about Aang searching for more airbenders and mastering the sub-elements. There could also be a big change too like maybe there will be a new war between all those subelement nations and the other nations. The subelement war is not very likely. But if they make another book, they will probably have another big war thing to keep the action going.
Will Aang and the gang return to the South or North Pole? Probably not.
We get to see the Eastern Air Temple later, but what about the Western Air Temple or the northwestern Air Nomad Territory? Probably.
Is there another Airbender alive? Probably. There could be a group of secret airbenders that could help Aang in the final battle or something.