This is going to get me yelled at for this take. I'm going to talk about Aang decision with Ozai. I believe Aang should've killed Ozai and learned Energybending later. There are two reasons I believe this.
First, I believe it would've been a great contrast to Aang's character and shown that things won't always go how we want them. Or another point is that you have to make decisions even when it is against your morals, especially for the avatar. We have seen it many times before from other avatars and in korra. Whenever an avatar makes decisions based on their emotions and selfishness it backfires at a later point either in their lives or the world.
Secondly, I believe it would've hit harder if Aang learned at a later time after he already killed Ozai. Imagine the character development Aang if would've had when he killed Ozai. Aang learning that he could've kept his morals instead of going against them would've been a cool and interesting plot point.
I have many gripes with the ozai as aang fight and the glaze aang throughout thw fight. That will be for a later time, though.