Well I would love to see 4th season of avatar. There are books of every single element but air? that would just not be right... Anyways I have always loved those bit sad + bitter-sweet endings. Dont know if this would be something u would like but if I were given the change to write the 4th season it would be something like this:
Fire lord is defeated. Zuko has taken the throne and is now new ruler of the fire nation also as he has now friended with aang and has seen the error of the old ways he is now pulling all soldiers back. Yet there is still a serious unbalance with in the elements. So aang will set out on a journey with his friends to restore it. In the end of the season the balance has nearly been restored, yet one thing remains... as avatars are still humans and no human is supposed to live more than 100 years, aang himself is bringing unbalance to elements. So he must leave the world and be reborn to continue the eternal cyckle of avatars. Taking its place is an airtemple where airnomads are once again living thanks to aang, aang will come there with his friends to move on. Aang goes to meditate and as katara, toph, sokka and zuko are watching he slowly vanishes. Then they go and mount on appa and fly away... last scene would show a young baby with a birthmark (not tattoo) shape of an arrow in his/her head. A gift to aangfrom the spiritsthat all the future avatars will carryto honor the great avatar aang who brought the balance and restored peace in a time ofgreat chaos.