Alright as of the last nights episodes it is fair game of what will happen in the next book here is what I think will happen

After the fall of Ba Sing Se the gang is in a depressing mood and trying to find some good in their situation. One day they are all getting something to eat and they overhear some people talking about thow the firelord himself is going to be visiting the conquored city to declare the war in the earth kingdom over in a few weeks. Realizing this maybe their only chance to take out Ozai the gang must find any firebender who is willing to train Aang so that they can end this war once and for all.