Often, the Rinnegan in Naruto fanfiction is used to create unique power-ups for Naruto. Writers may show Naruto developing new jutsu specifically related to the Rinnegan. For example, he could combine his Rasengan with the Rinnegan's gravitational or absorptive powers. This not only makes for an interesting read but also allows for new and exciting battles against both old and new enemies. It can also be used to change the dynamics of his relationships with other characters, as they might view him differently with such a powerful dojutsu.
In Naruto fanfiction, the Rinnegan in Naruto is often portrayed in really cool and powerful ways. Some fanfics might give Naruto the ability to use all the Rinnegan powers to their fullest extent right away, like being able to effortlessly summon the Gedo Statue or use the Deva Path powers without any drawbacks. It's a way for writers to explore new storylines and make Naruto even more of a badass than he already was in the original series.
Another possibility is that it's a result of some strange chakra experiment gone wrong (or right). Scientists in the Naruto universe are always dabbling in chakra research. Maybe an experiment meant to enhance her natural abilities instead gives her the Rinnegan, and she then has to learn to control it quickly to avoid being consumed by its power.
If Naruto got the Rinnegan, he might become more cautious. The power of the Rinnegan is so great that he would realize he needs to be more careful with it. He wouldn't be as brash as before.
In Naruto fanfiction, the concept of 'Naruto Demon' can be portrayed in various ways. Sometimes, Naruto might be depicted as having some sort of inner demon that he has to struggle with, like a dark side of his chakra or personality. It could also be that a demon is fused with Naruto in the fanfiction's alternate universe. This can lead to new powers and challenges for him as he tries to control or come to terms with this new aspect of himself.
Well, at first it might cause some strain. His sister may feel overshadowed by Naruto's new power. But as time goes on, Naruto could use the Rinnegan to help his sister discover her own latent abilities. This would transform their relationship into one of mutual growth and support. He could use the powers of the Rinnegan to train her in unique ways, and she could in turn offer him emotional support, making their bond even deeper than before.
It might be related to a connection with some powerful entity in the Wave arc. For example, if there's a secret shinobi from the past whose power is tied to the Rinnegan and Naruto somehow forms a bond with that power source, it could lead to the awakening. Also, the intense emotions and challenges he faces in the Wave arc might be a catalyst, pushing his chakra to a new level and awakening the Rinnegan.
Some fanfictions suggest that Naruto achieves the Rinnegan as a result of intense training in a new form of chakra control. He might discover a way to tap into a deeper level of his own chakra, which has been dormant all this time. As he delves deeper into this new form of chakra manipulation, his eyes start to change. Over time, with continuous practice and facing numerous challenges that push his limits, his eyes finally transform into the Rinnegan. It could also be that he has a genetic predisposition that was activated by a specific event in the fanfiction, like coming into contact with a particular type of energy or meeting a certain character.
In some fanfictions, Naruto might awaken the Rinnegan through a powerful jutsu overload. For example, if he combines a super - charged Rasengan with some ancient and forbidden chakra - based techniques. This could potentially trigger a genetic or chakra - system shift that results in the awakening of the Rinnegan.
Well, it could be due to a genetic anomaly that was dormant within Naruto all along. Maybe his Uzumaki and Namikaze bloodlines combined in a unique way as he faced extreme challenges in the fanfiction, which then led to the unexpected evolution of the Rinnegan. For example, if he was exposed to a strong and strange form of chakra during a battle with an extremely powerful enemy, this could be the catalyst for such a transformation.
One implication is that he becomes much more powerful. The Rinnegan gives him access to powerful abilities like the Six Paths techniques, which would change the power dynamics in battles in the fanfiction. He could now face stronger enemies more easily.
In some fanfiction concepts, it might be due to a unique chakra interaction. Maybe the Kyuubi's powerful and ancient chakra somehow triggers a latent ability within Naruto that results in the manifestation of the Rinnegan. It could also be part of some kind of prophecy or destiny within the fan - created world of the story.