Even if there making a fourth season as I type or they are never going to make one this may strike an idea to start one.
Okay we now two main things there are no more airbenders and the last season ended as a cliff hanger even though it ended well. We also know that the avatar is the only one that can enter the spirit world and that we have only seen good spirits. Also that each avatar in some point of there life before they die face a challenge.
Heres where my thought comes in what if the reason we don't see bad spirits is because an avatar long ago before the fire nation went loco locked them away somewhere in the spirit world. Because these evil spirits that since hurting you with there own strength is against the law of the spirit world they would come to you in dreams and manipulate you into doing something whether it be to yourself or someone else. The only way for these evil spirits to be released is by the chain to be broken. When I say chain i mean avatar chain. What if all you had to do to stop the cycle of avatars is whip out one single element. What if one of theses evil spirits escaped and disguised himself as human. He knew the air nomads were a peaceful people and would not fight unless they had to but still they would not try to kill. Know all he needed was someone to start the war and the avatar to die and be reincarnated in the as the air avatar so that the army would have an excuse to whip out the air nomads. The water tribes were in a way like the air nomads and would not be easily manipulated. The Earth kingdoms were stubborn and would do very little to help him though the air nomads would be defenseless against them. The fire nation on the other hand may be easy to manipulate for the fire lord is friends with the avatar. So he went to the Fire nation spending his days visiting the Fire lords dreams and filling his head with silly and evil thoughts of expanding his territory and invading other colones while the avatar was training. By the time the avatar had returned from his training and become a master in all elements it was to late the evil spirit had all ready got to the Fire lord. After the last fight with the avatar and the Fire lord the evil spirit was beginning to give up on the Fire nation and going to move to the next element when the opportunity to kill the avatar came. The Fir lord whipped out all airbenders or so he thought. What if some of the airbenders like the children and teens where escaped to an island were no maps lead to except one that is made into two copy's. So the airbenders along with people that over the years have crashed there. While a few very few airbenders live in nations around the world waiting for the return of the avatar so that the map keeper can take the avatar to the air nomads and send the evil manipulative spirit back to its cage in the spirit world. The evil spirit knows that all the air nomads are not dead so he searches for them with his manipulative powers.
So Aang has just defeated the Fire lord but unfortunately the fire lords followers still drag the war on. So azula should be in prison and the evil spirit frees her because of her ability to make her fathers followers fallow her and she will not think twice about killing the air nomads.
The map keeper a young 13 year old girl also a air bender comes to Aang to tell him of the others and how he needs to help them. She also warns him of the fact that azula knows and now is looking for the map and killed the few people protecting the map. But what azula did not know was this air bender Opiekun "Keeper." Did not have the map on paper but hiden in her skin and she can hid it and make it visible to who ever she wants. Unfortunately there are always two map keepers and the other one just so happens to be a air nomad that is very evil. After all not all air nomads are good. Azula does not know where this other map keeper is or that there are even two. Just that she needs to find this map and kill the nomads along with the avatar.
Oh and prince zukos mom crashed on the air nomads secret island and is now living there waiting for the day she can return to her baby boy zuko.
So the whole gang has another adventure to go on.
I know I have probably left major details out that should be covered so tell me if there are any problems other than the fact that they probable won't do another season.