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.
Another plot could involve a common enemy that threatens both the Naruto and Avatar universes. Naruto and the Avatar would have to team up, along with their respective friends and allies. They would need to combine their powers, like Naruto's rasengan and the Avatar's element bending, to defeat this powerful foe.
One idea could be that the characters from Avatar: Pandora discover a portal to the Naruto universe. They are amazed by the ninja abilities and chakra system. Some Pandora characters might try to learn ninja techniques, while Naruto characters are intrigued by the unique flora and fauna of Pandora.
Perhaps the nine - tailed fox in Naruto and the Avatar state could have some sort of connection. In the fanfiction, the two powers could interact in unexpected ways. For example, when Naruto enters his nine - tailed fox mode, Aang's Avatar state could be affected, or vice versa. They might have to learn to control these interactions to save both their worlds from a dark force that wants to use this power connection for evil.
In a naruto x avatar fanfiction, there could be a scenario where the Akatsuki from Naruto's world teams up with some of the villains from the Avatar world. This would force Naruto and the Avatar (whichever one it may be) to put aside their differences and form an alliance with other characters from both worlds to stop this powerful and evil coalition.
One idea could be that the Avatar characters gain the ability to use magic when they enter the Harry Potter world. For example, Aang could use his airbending skills along with magic spells to fight against the Death Eaters. Another could be that Hogwarts students are sent to the Avatar world for a special exchange program, and they have to learn to bend elements instead of casting spells.
One idea could be a crossover with another fantasy world where Korra has to learn new bending techniques from that world's magic users. This would bring a fresh dynamic as she struggles to adapt to different power systems.
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.
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.
A Naruto x One Piece crossover would also be cool. Naruto could end up on the Grand Line and team up with the Straw Hat Pirates. His ability to create shadow clones could be useful in battles against other pirates, and he might learn some new skills from Luffy and the crew.
A plot could be centered around Zuko from Avatar joining the Straw Hats for a while. He could teach Sanji some fire - bending inspired cooking techniques. Meanwhile, Nami might use her knowledge of weather to help the Avatar characters understand their own natural elements better. And Usopp could create some cool gadgets inspired by the technology and bending in the Avatar world.