A fanfic could also explore what would happen if the Sharingan was combined with bending abilities. A character like Sasuke having the ability to bend fire in addition to his Sharingan powers would be extremely powerful and could change the power dynamics in the Naruto - Avatar crossover universe.
One idea could be a crossover where Naruto characters learn bending techniques from Avatar characters. For example, Naruto might try to master water bending, which could be a really cool concept as he already has control over his chakra, and adapting it to bending could create some unique fight scenes.
One idea could be Naruto having an encounter with a death - like entity that grants him new and dark powers. He might struggle with the morality of using these powers while still trying to protect his village.
One idea could be a story where the character with the Rinnegan avatar has to navigate a world where the rules of chakra and bending are combined. For example, they might use the Rinnegan's powers to control different elements in a way similar to how avatars control them in the Avatar world.
Maybe the Avatar, in his journey to master all the elements, travels to the Naruto world. He could learn some ninja techniques and incorporate them into his fighting style. The Avatar could also teach the ninja about the power of bending, which could lead to some really unique team - up strategies among the characters.
One idea could be exploring the backstory of a minor character from the Avatar world in the sequel fanfic. For example, a Na'vi who was on the fringes of the main events but has their own interesting journey. Another could be a story about the first human - Na'vi hybrid, dealing with identity issues and being a bridge between the two cultures. Or perhaps a fanfic that focuses on the restoration of a part of Pandora that was damaged during the events of the movie, showing how the Na'vi and any sympathetic humans work together to heal the land.
One idea could be to explore the backstories of the avatars in the brotherhood. Maybe they all come from different worlds or cultures and how they came together to form the brotherhood. Another could be a story about a new avatar joining the brotherhood and facing challenges to be accepted.
Perhaps the Akatsuki from Naruto somehow get involved in the Avatar world's politics. Their goals of collecting tailed beasts could be translated into a search for some powerful Avatar - related artifacts. This would stir up a lot of chaos in the Avatar world and force the Avatar and his friends to team up with some Naruto characters to stop them.
One idea could be a story where the main character has the power of the Lightning Avatar and is sent to the Naruto world to help in a great war. They could use their unique lightning - based abilities to turn the tides of battle against powerful enemies like the Akatsuki.
One idea could be Naruto somehow accessing an Avatar State similar to the one in 'Avatar: The Last Airbender'. Maybe he discovers an ancient jutsu that gives him this power. He could use it to defeat a powerful new enemy that is immune to normal ninjutsu.
One idea could be to have the Voltron pilots end up in the Avatar world. They could learn bending techniques and use their Voltron skills in combination with them. For example, Keith's swordsmanship might blend well with waterbending moves in a fight.
Perhaps Ben, while exploring a new planet in his travels, stumbles upon a world that has a very Avatar - like culture and power system. He has to learn to use their ways in combination with his alien forms to survive. For example, using Heatblast's heat powers in a more refined way similar to firebending.