Thinking of writing a fic? Great! Here are some ideas you can use to keep your fic from straying into the cliche...assuming that's what you want to do.
Stories focusing on the spiritual world.
Stories focusing on minor one-ep charrys.
Stories focusing on the war (ex: A fic starring Katara/Sokka's dad)
Stories focusing on past Avatars.
Stories focusing on the happenings at the temples.
Stories where Ozai WINS.
Stories containing vast amounts of political intrigue.
A one-shot where Zuko stops at a harbor and doesn't fall in love with a street urchin.
Anything focusing on Lieutenant Jee or other members of Zuko's crew.
Stories focusing on Bumi/Iroh/Gyatso/Gran-Gran's childhood.
Romance stories that pairs Katara with someone other than Aang or Zuko. (Like Haru. Or Jet. Or the firebender performer from The Deserter.) Good La, it's been written!
A friendship story between Zuko and Sokka.
Anything about the Yu Yan.
Stories focusing on Iroh's relationship with Zuko's mom.
Stories about Iroh's siege on Ba Seng Sai.
A documentation of the Lotus Tile's travels.
Romance stories between the older generation of Avatar: eg, Pakku/Kanna, Iroh/Kanna, Bumi/Wu, etc.