I've actually made progress on my current story draft, go me! I've gotten past the part actually giving me trouble, so I'm gonna celebrate with another bit of lore from my LUxATLA AU.
Wind! Baby boy! Tied with Four as my favorite of the heroes! This boy most definitely started as a water bender before discovering he was the Avatar at the ripe old age of twelve.
Now, honestly, i don't think Wind's first story would change all that much. The Helmaroc King is most definitely just a "noraml bird" in this version of events, and the King of Red Lions wouldn't be able to just be the boat, but... very little else would actually need to change all that much.
No, I think the real changes would have to come in the form of Phantom Hourglass. First of all, Wind's adventure can't happen in a dream world, and it doesn't make that much sense for the entire adventure to take place in the Spirit World. So here's the pitch:
Instead of Wind and Tetra getting stolen away to a new world, Wind and Tetra get separated from not only their crew but each other during a great storm that had been kicked up during the fight between the Ocean King and Bellum. Wind gets found by Ciela, one of the ancient shard fragments of Farore.
Wind still meets Oshus, who is not the Ocean King but is instead a guru that knows far too much about what is happening between the Ocean King and Bellum. Linebeck is still a non-bending treasure hunter, Tetra is still trapped by Bellum's power, and the rest of the NPC's in game would be practically unchanged.
Leaf and Neri are fragments of Din and Nayru respectively, just like Ciela with Farore. This has nothing to do with Bellum or the Ocean King. This is due to what happened before the avatar cycle even existed; Hylia would have had the same thing happen to her if she had not irreparably damaged herself the way she did.
Another change is that the Sand of Hours does not control the flow of time like it does in game. That power does not exist outside of the spirit world in this story. Instead I'm going to say it's a lot like kinetic sand mixed with magnetite. This makes it a lot harder to control than normal sand but also keeps it's shape better than normal sand can. This allows Wind to effectively stop enemies in their tracks without risk of his foes being able to easily bend their way out, depending on how much he has at any given time. This also means he can add the sand to whatever weapon he wields to add to it's shape/weight.
Phantom Hourglass would also be when Wind is able to control the avatar state instead of continuing to accidentally trigger it. This is saved for the final battle against puppet!Linebeck but it happens!
TLDR; Wind invents a more advanced form of sand bending before he even masters the avatar state.