One idea could be that Azula somehow gets transported to the Naruto universe and is given the power of the Rinnegan. She might use it in her own cunning way, perhaps to gain more power over the Hidden Villages. She could manipulate the new powers with her already devious nature, creating a new and dangerous force in the Naruto world.
In a fanfiction, the Rinnegan's powers could be merged with Azula's firebending. Imagine Azula being able to use the Rinnegan to enhance her fire attacks, making them more powerful and unpredictable. For example, she could use the Preta Path to absorb chakra - like energy and convert it into more fuel for her flames. This would make her a formidable opponent not just in the Avatar world but also if she were to cross over into the Naruto realm.
One idea could be a crossover where a character with the Rinnegan from the Naruto universe somehow ends up in the Percy Jackson world. Maybe they use their powers in unique ways, like using the Rinnegan's ability to see chakra as a way to detect the demigods' special powers which could be seen as a form of energy similar to chakra.
One idea could be that a character from the Naruto universe with the Rinnegan somehow gets transported to the DC universe where the Justice League exists. They could initially be seen as a threat, but then prove their worth by helping the League against a common enemy. For example, using the Rinnegan's abilities like the Six Paths powers to combat a powerful villain like Darkseid.
One idea could be a crossover where a character from Young Justice gains the Rinnegan somehow. Maybe through a strange experiment gone wrong that links dimensions. They then have to figure out how to control its powers while still being part of their team in Young Justice. It could lead to some interesting power - up moments and new story arcs.
One idea could be a character from the One Piece world somehow obtaining the Rinnegan. This character could then use the Rinnegan's powers, like the ability to see chakra, in the context of the One Piece world. For example, they might be able to detect the 'life energy' of Devil Fruits or sense the hidden strength of characters like Luffy's Gear forms. It would create a whole new dynamic in the One Piece universe with this new power.
One idea could be a story where a new character is born with the unique combination of Rinnegan, Sharingan and Mokuton abilities. This character could struggle with the power and the attention it brings from different ninja clans. They might be seen as a threat or a savior, depending on who's looking at them.
If Azula had the Rinnegan in fanfiction, it would change her character arc significantly. She might start to view power in a different way. With the Rinnegan, she could potentially gain access to knowledge about different worlds, like the Naruto world. This new knowledge could either make her more power - hungry or cause her to have a change of heart and use her powers for more noble purposes. She could also use the Rinnegan's ability to revive the dead to bring back someone important in her life, like her mother perhaps, which would add a whole new layer of complexity to her story.
One idea could be a crossover where Naruto, with his Mokuton abilities, somehow ends up in the Avatar universe and meets Azula. Maybe he tries to use his Mokuton to counter her firebending in a big battle. Another could be a story where Azula discovers a way to use Mokuton-like powers through some dark magic and Naruto has to stop her from misusing it.
A fanfiction could revolve around a young ninja who is born with both the Sharingan and the Rinnegan. He has to hide his powers from the world as he is hunted by those who want to use his powers for evil. As he grows, he meets Naruto and they form an unlikely alliance. Together, they train to master his unique abilities and fight against the forces that seek to destroy them.
One idea could be a story where Naruto somehow acquires the Rinnegan and combines its powers with the Susanoo. Maybe he has to face an extremely powerful enemy that can only be defeated by this new, super - powered combination. He could start by accidentally awakening the Rinnegan during a battle and then discovers that he can merge it with his Susanoo - like form.
One idea could be that Fuu discovers a hidden power within Naruto's Rinnegan that can open a portal to a different dimension filled with ancient ninja secrets. She then teams up with Naruto to explore this new world and face the challenges within.