I would assume it's game over for everyone if Aang does not stop the comet. Though I want to think that he's going to die in the Avatar State. With the Fire Nation stopped, there will be no more need for an Avatar.
Ozai is bound to die I'd say. 95% sure he will. Azula will probably either die or be imprisioned. As much as I love Iroh, he's got the wise old mentor thing going for him. They don't tend to live. Though if he does, I would think he would become Fire Lord, with Zuko as his heir.