During the northern water tribe arc, imagine if none of the gaang try to take zuko with them who is a fire bender unconscious in the snow during a blizzard?
So most things go as normal but iroh and fire nation learns that the avatar and his companions left and most likely left the prince (sure he is a banished and exiled prince, but still a prince) and after aang lets the ocean spirit kill the entire fleet that they cant defend themselves against? They would be angry if they were defeated by humans but a spirit? That is plain cheating. And shows that the avatar and spirits are being used against the fire nation and the others are glad.
(Spoilers but zuko did live and manage to live incognito or traveling in the earth kingdom under an alias... He is tired and didn't have enough time to tell uncle or anyone else given the posters)
So after everything the gaang and the world think they killed the fire nation prince. And set the ocean spirit on the fire nation navy.
Iroh also thinks that. He lasts long enough to get to earth kingdom but now 'lost' the last thing, his second son. And he did so to the water tribe and the avatar with his companions. The avatar is the bridge but he also bridged and set the ocean spirit on the fire nation.
So the fire lord is happy (inside) that his son can be used for propaganda. His son was so close to getting rid of the threat the avatar represents and he was to bring alive to him. However, the avatar killed the crown prince and next in line as well as setting loose the spirits against the fire nation, which is what the fire lords and zuko died trying to prevent.
Azula is now given all the resources needed to capture or KILL avatar and friends before they bring the evil to the capital.
All of fire nation will hear of what they did and now will be on a war path. You thought before was a war path? Haven't seen a real one. Their prince is dead, killed trying to prevent what happened. The avatar, bridge between spirits and humans set a spirit, the very ocean itself, and destroyed and killed a large navy. They see the princess as the next ruler in line and going to stop the avatar.
Also the water tribes. Hakoda and the southern water tribe men are fighting but they prob weren't a high priority. Now they are. And guess who is just women and children in their home? Now they have hostages. While the northern water tribe will try to go and help, they have to go near fire nation ocean territory. And given what just happened, they are going to be pissed. The avatar is gone to earth kingdom... Fight the northern water tribe on the new moon while he is gone.
Then they can focus on earth kingdom. Well, azula is with her resources that arent fighting and destroying water tribe.
She meets the avatar and group and starts the fight by talking about zuko. Her older brother, crown prince who couldn't go home unless he captured and presented the avatar to their father. But the avatar killed him. While pretending (and getting unnecessary feels that almost make it true) to care, she suddenly stops and thanks them. She pronounces that since zuko isnt alive or unbanished, she is now crown princess and next in line for the throne. She wishes she was there while zuko breathed his last and also sad that she or father couldnt be the ones to kill him.
Also maybe iroh is with them, giving information. He would have liked to help the avatar but that was before they killed his nephew. His second son. Also the dragon of the west came out of retirement to burn down ba sing se.
Stuff happens but the water tribes and earth kingdom get to know how it feels when the fire nation is literally breathing down your throat and trying to conquer. Fire nation was doing so without interference of spirits or avatar but then water tribe called the avatar and ocean spirit to do their work for them because they would have lost and the fire nation cant fight fairly against that. Well now they dont have to fight or win fair because if the others do that, then so can the fire nation.
Stuff happens but then zuko happens. Look, he has been hiding and resting from the world and didnt know he was dead. So he goes to fire nation and hey, prince made it! He survived the avatar and his companions killing him! Along with the cold as well as the ocean spirit.
Don't know where this is going but its a 'consequences if zuko "died", but not really' and also because I dont want him actually dead and he has survived worse.