for sure this is one of the best games on this site, it's in my top 3 easy, but if I had made or was going to make a mod like the prince zuko one, I would make some changes, for example:
-I wouldn't do the route system, instead I would give more options for the player to choose to follow or not, since a lot of people miss several good scenes by only playing one of the routes.
-I would change the order of the books and hold the story more faithfully, for example:
book 1 : it would take place in the north pole, so the protagonist would have katara and yue as romantic interests, in this route we would also have the beginning of the avatar state training, entering the spiritual world and having sex with koh, the wife of kuruk and kya (mother from katara).
book 2: i would follow toph's story/route and leave it as is, just add more characters and put in the first gay scenes of the game, with jet and haru.
book 3: it would take place after the events of the last war, with zuko being lord of fire and azula trapped (as happens on the slave route), from there the player could interact not only with ty lee, mai and azula, but also with Ursa , Kiyi, koh's mother, maybe Ozai and Iroh and the girl aang interacts with as Kuzon.
book 4: this one would be worth splitting and more than one route, to be able to take advantage of concepts and characters from several seasons, I could start with the protagonist in Tenzin's body (interacting with the characters from book 1 and 2) and then the normal protagonist , to see the results of what you did in the previous route and in addition, interact in the spirit realm with Raava and the earth queen and the red lotus womens.