I have many problems with the world of avatar but this one stands out to me often:
The fire nation should have and wouldn't have ever accepted Zuko as their ruler. think about it. for 100 years the fire nation have lived in a singular way. their culture and history and identity as a nation was all altered and the people for the most part lived in this willingly. the fire nation genuinely thinks the avatar and everyone else are the bad guys.
Where does Zuko come in? well firstly the officials would see zuko as weak given he did not fight an agni kai and disrespectful for the other reason. he was publicly humiliated and mutilated and dishonored and BANISHED for three years. then they didn't even get 3 months with him back before he willingly betrayed them again. he attacked their beloved ruler and went to join the avatar to destroy their empire which they honestly believe is justified.
Then when he does just that and defeats who is to them, the rightful ruler of their nation (azula) they just allow him to rule them. and it takes a year for anything to really pop off and thats only because of the colonies. the Ozai restorationists don't really move for a couple of years and they're mostly made of noblemen and a very small amount of them at that. and they only move under azula's orders, not their own. but for the most part, Zuko has the complete obedience, loyalty and cooperation of his entire kingdom with little to no resistance.
Sure it could be that aang enforced it but there's one problem 1) aang himself is not scary or intimidating enough (or imposing) to enforce that kind of radical change in such short amount of time without force (Like kyoshi) -- like it took kuvira 4 full years to bring a chaotic nation back under control and the bandits were only a problem for about a week to a month at best before she left. Zuko took control of a nation that not only hates him but is so culturally opposite to how he now thinks and acts that its insane.
There would be a lot of infighting, assassination attempts and maybe even a civil war if not an outright call for zuko's resignation. also they would not willingly keep ozai in prison or azula for that matter. they are loyal to them both. sure ozai has no bending and maybe that could alter the way they see leadership but azula is still kicking and her mental state wouldn't matter to them since it didn't before.
In short. even if aang could enforce such a radical change, it would cause amazing levels of consequences.
