Seeing as there probably will only be three seasons of ATLA, I tried to come up with some possible Avatar spin-offs because it would really suck if there was nothing to look forward to once the show has its finale (Not like these will ever get made, but a guy can hope, can't he?).
Avatar: The History of Aang
Basically, this is a story beginning with Aang's birth and ending when he is frozen in the iceberg. This will let us know the answers to any questions that are still open, especially regarding Aang's parentage; how the Air Temples interacted with the world, the other Temples, and the Air Nomads not in Temples; and how the Air Nomads lived their lives. We could also see Aang traveling across the world and interacting with his friends, such as Kuzon and Bumi (We might even meet Fire Lord Sozin, the then-Prince Azulon, and Guru Pathik). It would give us a chance to see how the world looked a hundred years ago, and we would get to see everything before it becomes ravaged with war.
Avatar: Mending a World
This would be the tale of the recovery of the world after the war. It could be told through older versions of the current characters or new characters, but either way we would get to see what happened to the main characters and main secondary characters, as well as a few of the minor secondary characters (Cabbage Man and Foaming Mouth Guy would be definites. Also, Aang might have died when he defeated Fire Lord Ozai, so the story's main character might be the new Avatar. Character's children might be important characters).?? We would get to see how each nation is doing (Maybe the Air Nomads will make a recovery?), how they feel about one another, and especially what the Fire Nation's role in the world is. Tensions will still probably be high and skirmishes are a major possibility.
Avatar Chronicles (Note: Not my idea, one I saw on the boards a while back, I forgot who came up with this and I apologize)
This would be about tales from the lives of past Avatars. It would give us details about that Avatar's life and the times they lived through. Seasons would be broken into four consecutive Avatars, one of each element. It could either start with the first Avatar and end with Roku or vice versa, the latter giving us some interesting backshadowing. Each five episode block devoted to an Avatar would follow them through their training and effect on the world, each Avatar fixing?? what the previous Avatar missed or maybe even caused. It would give us a chance to see how other Avatars acted and interacted with each other in the Spirit World, letting us recognize different aspects of Aang's personality coming from different places.
Avatar: World at War
This would be about the years between when Aang was frozen and when he was found by Katara and Sokka. We would get to see the major battles and the first responses to the war. It would give us light on how the Air Nomads responded to the war and were annihilated, as well as what the Southern Water Tribe was like. This would give us a chance to learn about the Royal Fire Family as well as Katara and Sokka's family. We would get to see how all of the major cities became the way they are now. Also, we would see what everyone thought about the Avatar.
