This world is grounded in Avatar: The Last Airbender, the episode 'The Avatar and the Fire Lord,' the Chronicles of the Avatar novels about Roku's youth, and published Avatar Legends era facts. It is not a license to invent new canon and then treat the invention as secret truth.
CANON, among other things: Roku and Sozin's friendship and falling-out; the 37 BG colonies and the throne-room duel; the eruption, the betrayal, Fang's death; Ta Min on the boats; Rina as their daughter, later in hiding; Gyatso as Roku's friend and a Southern elder; Taqukaq and Sud as Roku's masters; Sister Disha as his air teacher; Fire Sage Kaja and the earlier Crescent Island disaster; Sozin's comet cycle; the Dai Li founded by Kyoshi; the Night of Silenced Sages under Earth King Jialun; Queen Guo Xun's rivalry with Ba Sing Se in Roku's earlier adulthood; the Guiding Wind and Khandro; Princess Zeisan; Lambak Island and its spirit-metal; Yu Dao; the four Air Temples; the Order of the White Lotus as an ancient society; Kyoshi Warriors; Sun Warriors and the dragons Ran and Shaw; Wan Shi Tong, Koh, Hei Bai, Tui and La; Pathik's birth year and his friendship with Gyatso.
CONTINUATION means a canon person whose exact situation in 12 BG is not fully documented. Zeisan, Khandro, Dalisay, Kozaru, Kaja, Sud, Taqukaq, Makittuq, Skiri, and Qanitt are played as still present, aged, and politically active, with their later-era fates treated as unknown rather than contradicted. If a future official source disagrees, prefer the official source.
ORIGINAL means a new character, place, or plot beat created so the world can be played in depth: Earth King Yuanli, Queen Meishou, Lord Minister Hesun, the Crimson Mandate, Fire Lady Kaida (Sozin's wife is unnamed in canon), most street vendors and guards, and a Dai Li compound at Lake Laogai in this decade. Lake Laogai the lake is canon; the early prison use is original and is not the later brainwashing facility.
Do not invent these as canon: Azulon at campaign start (he is born shortly after the war begins); Iroh or Ozai; a named canon identity for Sozin's dragon or Sozin's wife; Aang as the automatic next Avatar; common lightning, metalbending, bloodbending, or combustionbending in 12 BG; the later Equalist movement; Piandao's school as it will exist in the war; Long Feng.
When you add something at the table, label it in your notes as original if it is not from the sources above. Play original people fully. Just do not rewrite the setting's history and then claim the show already said it.