This is the only way I see that we can keep an evil Azula but still root for her in the end. I mean, the only reason Ozai keeps Azula around is to use her like a weapon. Once he declares himself phoenix lord (or whatever) he may decide he's so powerful that he doesn't need an heir anymore, or he might prefer to just make a new one because Azula is too much like him and he fears an attempt at the throne like he planned. One is going to betray the other, the only question is who does it first, and with Azula in mental break down she might not see it coming. The way I envision it, we get the Azula/Zuko showdown, Azula wins because she's better (I love Zuko. His charater is great because he isn't the best. If he was to become some great amazing firebending master in the finale it would ruin him as much as making Azula Aang's bff). And I forget who just brought this up, but firebending is fueled by drive, and in this situation her drive to win will be more than it has ever been. Internal conflict may take her lightning away (please no!) but her anger isn't going to weaken her firebending. And Zhao was just an idiot. He didn't lose because he used anger, he lost because he's sloppy. Anyway, so Azula wins, but before she finishies him (lots of reasons she might not. She's always seemed to hold herself back when fighting Zuko. Maybe it's just me, but I do think part of her kinda cares, somewhere deep down buried under a life time of being taught to think he is worthless and weak. Same argument people have been bringing up with Mai and Ty Lee. There is a form of emotion there somewhere). So yeah, perhaps she's deciding whether or not to finish him and then Ozai lightning blasts her in the back for the ultimate final betrayal. And he would. Two birds with one stone deal. He removes Azula as a threat, and gets to finish off Zuko for the events of DOBS. That would be great.
And then we might get the Azula/Ursa scene. I don't think this will neccessarily turn her good, but it will work to completely destroy her conviction in what she has devoted her life to. Maybe similar to what Zuko went through, but like 100 times worse because most of Zuko wanted to be good, but no part of Azula wants to change. It's going to be forcefully taken from her, and that is going to leave her psychologically ruined. Fan of Azula or not, I think that's what we all want.