In fanfiction worlds, the rules can be bent. Itachi could be written as having a connection to the origin of the Rinnegan that no one else knew about. He could then use this connection to give Naruto the Rinnegan. This might be part of a larger story arc where Naruto needs the Rinnegan to defeat a new, extremely powerful enemy. The author could build a whole new mythology around this transfer of power, exploring the implications for Naruto's character development and the overall Naruto -verse.
In this kind of fanfiction, the author might come up with a convoluted plot. For example, Itachi could have discovered an ancient technique that allows him to bestow the Rinnegan on Naruto. It might involve sacrificing his own ocular powers in a way that defies the normal rules of the Naruto universe. This would be a very creative and out - of - the - box idea, but that's the beauty of fanfiction.
Naruto's perspective on the ninja world could change. With the Rinnegan, he would see things in a different light, both literally and metaphorically. He might have to deal with new enemies who are attracted to his new power or new responsibilities that come with such a powerful dojutsu. His relationships with his friends and allies could also be affected as they may view him differently now that he has this extremely powerful ability.
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.
In fanfiction, almost anything can be possible as it's a realm of creative interpretation. However, this concept goes against the original nature of the Naruto series where Naruto is male. But in fanfiction, some authors might use magic or some unique plot devices to make such a situation happen, but it's not in line with the canon.
It could also be a situation where Naruto's connection to Hagoromo is deepened in the fanfiction. For instance, Naruto might have a vision or a spiritual encounter with Hagoromo. During this encounter, Hagoromo realizes that Naruto is the key to bringing true peace to the ninja world. So, he imparts the power of the Rinnegan to Naruto to help him achieve this grand goal. This would be a more complex and involved process, perhaps involving Naruto learning about Hagoromo's past and his own role in the grand scheme of things.
In some fanfiction concepts, there could be a spiritual or mental connection established between Naruto and Nagato. Since Naruto has this unique ability to understand and connect with people on a deep level. Through this connection, Nagato might pass on the power of the Rinnegan to Naruto. It could be like a transfer of not just the physical power but also the knowledge and understanding that comes with the Rinnegan.
In some fanfictions, it could be through a strange and powerful jutsu transfer. Maybe Madara, on the verge of defeat, decides to transfer his Rinnegan power to Naruto for some unforeseen reason, like seeing a new path for the ninja world in Naruto.
In some fanfiction concepts, the chakra of the Kyuubi is not just pure fox chakra but also contains remnants of the Sage of Six Paths' power. Since the Rinnegan is related to the Sage of Six Paths, when Naruto reaches a certain level of power and harmony with the Kyuubi, the dormant Rinnegan - like power within the Kyuubi's chakra awakens in Naruto. This could be a result of Naruto's strong will and his journey to master the Kyuubi's power which eventually leads to him obtaining the Rinnegan.
In some fanfictions, it could be through a special jutsu or ritual. Maybe Itachi, before his death, transferred his Sharingan to Naruto in a unique way, perhaps using a forbidden technique that was known only to a few in the Uchiha clan.
He might become much more powerful. With the Rinnegan, he could gain new abilities like the Six Paths powers, which would make him a force to be reckoned with in battles.
One way could be through a powerful jutsu gone wrong. Maybe a jutsu that was meant to transfer Itachi's memories to Naruto accidentally transferred his Sharingan as well. In the world of fanfiction, there are often many creative and out - of - the - box explanations.