Aang/Redemption Arc of any kind
Aang is redeemable. I know that's weird coming from me because I hate that brat, but I have actually given him a redemption arc in stories before, and even when he's at his worst, I usually leave space for him to grow later on. That said, in order for Aang to grow, people have to treat him like the child he is. He has to face consequences for his actions (and inactions), and he has to take his disappointments on the chin. He cannot end up with Katara! I don't think a mature Aang would even want to be with Katara because he would recognize how he used her as a crutch and a shield. Here's the thing, I realize Aang is a child. Children are supposed to make dumb choices, and empathy is something they have to grow in. Aang's problem is his dumb choices are seen as "wise" and he never has to actually empathize with anyone. Not even his "friends".
One thing I'd love to see (and admittedly, I've never done this either) is him dealing with the trauma of losing his people. In the show, he doesn't ever address it again after that first blow up in the air temple. Sure, he mentions them sometimes, but he doesn't even bring it up when he's trying to show Katara he understands how she feels about Yon Rah. Like...? My guy, your people are dead, and you're comparing Katara's loss of her mother, her people, her culture to you losing Appa for a couple of weeks???? Aang is supposed to be wise? I don't see it.
He's 12. He grew up sheltered, and in a world without a war, and he's 12. He has no idea what he's doing. Everyone else in the Gaang knows the assignment and the stakes. Even Toph is better equipped, and she was sheltered, too. Aang is playing a whole different game from everyone else. He is over here playing Mario Kart while everyone else was on Fallout. And he's 12. He should be treated like he's 12. His ideas should be treated like a 12-year-old's. That's literally all it would take to redeem Aang and make him a less infuriating character.
