Listening to someone else watch the original ATLA again, just starting season 3, and there's this line where Aang suggests that it didn't just look like he died, but he DID actually die and Katara brought him back. And look, I don't know much about the lore introduced by TLOK, but this sounds like a really fun place to start an AU inspired by "Buffy the Vampire Slayer"'s take on the temporary death and resurrection of a cyclical chosen one: Aang is brought back to life and is STILL the Avatar, but his temporary death still also causes another Avatar to be born into the Southern Water Tribe (or Swamp People or refugees on some southern island, whatever).
Like, I know the Avatar is a reincarnation thing, but weird things can happen with spirits! Maybe this glitch in the cycle causes the spiritual equivalent of mitosis to happen to the Avatar and the spirit attached to them, so there's just two of them now! Maybe having two Avatars has some spiritual downsides, maybe not, it's up to whatever path you choose to explore in this AU.
I just think it would be really funny for post-canon 24yo Aang (and the Gaang and the White Lotus and all the spiritual / religious orders on the planet) to suddenly be confronted with a 10yo water tribe kid who is ALSO the Avatar.
A water tribe couple shuffle awkwardly up to Sokka, like, "Sooooo... we think our 10 year old daughter might be the Avatar...? She can, uh, bend all the elements and all that. Look at this shit." And Sokka is like, "Fuck, Aang is dead?!" Before he's like, "Wait, I saw Aang last week, this kid is way too old?!" And he has to hastily contact Aang like, "Hey, buddy, you're still alive, right?" And Aang can only reply, "Uhhh, last I checked, yes?!?!"
It's probably Sokka and Katara who put the pieces of the timeline together and figure out what caused this, because all of the other Avatars are probably initially like, "Yeah, I have no idea what's happening here. I'm you and you're me, but I'm also her and she's also me, and you and her are kind of the same person, but you're obviously different people now? Wow! Good luck figuring all that out, kid! You're really going places none of us have gone before! A real groundbreaker!"
Aang, through gritted teeth, "I am really not TRYING to do that, thank you!"