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An original concept for Aang had him coming from a Futuristic Sci-Fi world, with Aang being the last suvivor taking him 1000 years into the future as the world reverts back to a more ancient time. Momo would also be a robot cyclops monkey which sounds pretty cool (not that we'd ever want to change him).
Avatar Kurok is the shortest living Avatar, he died at the age of 33. This was caused by Avatar Yangchen, who neglected the spirits, because of this every time Angry Spirits invaded the world. When Kurok defeated them some of his life energy would be drained eventually killing him.
Uncle Iroh was voiced by Mako Iwamatsu for the first two seasons, but Mako passed away from cancer in the middle of production. Greg Baldwin, who'd been doing scratch dialogue and impressions of Mako for years, stepped in to finish out Book Three so the character wouldn't have to change entirely.
In 'The Tales of Ba Sing Se,' Iroh spends the whole episode quietly helping strangers before we find out why: it's his son Lu Ten's birthday, and Lu Ten died taking the city years earlier. Iroh sings 'Leaves From the Vine' through tears, and the segment closes on a dedication to Mako, who died just before the episode aired. It's considered by a lot of fans to be the saddest moment in the whole show.
Before Toph Beifong became the blind, sarcastic earthbending prodigy we know, the character was originally written as male. Bits of that original design apparently still float around the show in background characters, including the wrestler known as The Boulder.
None of the fighting in the show was made up from scratch. Waterbending is built on Tai Chi Chuan, earthbending on Hung Gar, firebending on Northern Shaolin, and airbending on Baguazhang, with Toph getting her own flavor pulled from Southern Praying Mantis. The show's animators actually studied footage of these styles to get the movement right.
Martial arts consultant Sifu Kisu worked on all 61 episodes of the original series. He'd physically act out each fight so the animators had real reference footage to draw from, which is a big part of why the bending choreography still holds up so well.
In-universe, humans didn't invent earthbending at all. The show's mythology credits the blind badger moles as the original earthbenders, with the two lovers who founded Omashu learning the art by watching the moles dig their tunnels.
Not every movement in the show is Chinese martial arts. The dancing in Season 3's 'The Headband' was actually based on capoeira, the Brazilian style that blends fighting with dance, giving that one scene a completely different rhythm from anything else in the series.
Master swordsman Piandao is voiced by Robert Patrick, best known for playing the liquid-metal T-1000 in Terminator 2. The writers apparently couldn't resist giving Piandao a throwaway line connecting the two roles, a little wink for anyone who catches it.
