Everbody thinks it's Zuko and even though it may be a plot twist, it's not exactly unexpected. Everbody thinks that Zuko would join Aang but I really can't see that happening. I think the stories will stay split only because they have to unless Azula, Mai, and Ty Lee decided to desert the fire nation and join the group, too. Zuko and the gang being together would just make it so much easier for Azula.
I don't think it will be Zuko or Iroh, just because they are main characters who happen to be firebenders who are not with the Fire Nation doesn't mean they're going to teach Aang.