We have an elderly Azula in a world where Ozai succeeded in conquering everyone. She finally hits her Villainous Breakdown with the realization that there is no way she can escape her impending death from old age, so she decides to go out in a blaze of glory, starting a war that will probably kill a good percentage of the world's population, and also set things up so that she'll be remembered as a hero who tried to stop that war.
Gian is a ruthless and sociopathicBounty Hunter, who doesn't care who is in charge of the world order or who his employers are, so long as he's paid for his services - which involve murder. He forces a rather meek young Earthbending slave to help him uncover the rebellion base, and kills her in cold blood simply because he had no further use for her. He's also revealed to be an Earth Kingdom man who essentially exploits his own people for profit, and admits as much to Jiazin without an ounce of guilt about it.
Magnificent Bastard: Azula and also Qing Xi. Both of them openly admit that they have no real trust in each other, and just trust the idea that there would be no advantage to either betraying the other.
Jiazin was originally supposed to be revealed as a descendant of Zuko and Mai through her mother's side. This was scrapped early on, since Zuko having descendants and Mai being pregnant were things Azula was unlikely to miss, and she would certainly have taken steps to... control... the problem. Tying into this, Jiazin's mother was supposed to be a much bigger character, but as this plotline was dropped, the focus shifted to her father (though I still regret not doing more with her).
Cheng wasn't supposed to last beyond the Empress Island sequence; she would have remained there and Shiyan taken up the hunt alone. As I wrote the scene where the Mistress gave them the task, however, it became obvious that as Cheng was the one who screwed up, she'd be put on the assignment to redeem herself. Besides, she gave Shiyan someone to talk to.
Chaiy was supposed to die during the rebel capture of Long Du Shi, not her father. However, while I knew I wanted a rebel leader to perish, it was less important which one, and with Yue's much more significant death coming at the same time- well, killing off two major female characters on top of each other didn't sit well with me. Jiazin's parents were also spared by the developing story; originally they were supposed to be killed after she went traitor, but it made more sense that two powerful nobles wouldn't be executed out of hand without the Empress's (who is a continent away) say-so, and the potential for an uneasy alliance with Yan Li and Chaiy felt much more interesting.
Aang was originally supposed to die and be reincarnated when the moon and ocean spirits were freed. However, I decided it was more fun to bring him back for a Big Damn Heroes moment at the climax and then subvert everyone's expectations with who really finished off Azula.
I toyed with having Kanoda develop waterbending abilities after the spirits were freed, but since with literally no one to teach him there was no way he'd get anywhere close to being useful with it by the end of the story, I decided it would be pretty pointless. More than a few surviving Water Tribespeople no doubt developed bending at around this time- but Kanoda wasn't one of them.
Chaiy was originally supposed to be a kind, supportive character with an edge that really manifested only when confronting the Fire Empire. This characterization survives to an extent in her earliest chapters, but after she first met Jiazin something about her firebrand revolutionary personality caught on.
Finally, Yuan was supposed to be ultimately revealed as the murderer of Kanoda's father, leading to a bit where he'd have to foreswear vengeance in order to get the spirit fish to safety. This just felt too contrived; Kanoda's father was killed by a nameless mook, and he'll likely never get closure.