I have some thoughts and ideas for Books 4 and 5. First is Book 4 will be based off of "Light". Aang finds out there are two more types of ways to bend, bending Light and Shadow. After Aang defeats the Fire Lord, the world falls into Darkness. Aang then recieves a message from Avatar Roku saying, "The world has two sides Yin and Yang, Light and Dark. When you defeated Ozai, you deafeated Darkness (this means Ozai was a "Shadow Bender"), the world needs to be balanced, without Shadow there is no Light." Roku goes on and tells Aang he must master "Light Bending" and "Shadow Bending" in order to bring balance back to the world.
And so the gang sets off to help Aang learn to "Light Bend" and Shadow Bend".
Oh yeah, and Zuko becomes the next Fire Lord (Azula is defeated by Katara), and he brings peace back to Fire Nation and the four Nations again.
I Know this Idea is kind of sloppy but, It came off the top of my head.