Well, in fanfiction, the rules can be bent quite a bit. It could be that the Kyuubi has some sort of hidden knowledge or power related to the Rinnegan. When Naruto forms a deep enough bond with the Kyuubi, perhaps through some intense emotional or life - threatening experiences, the Kyuubi transfers or awakens the Rinnegan within Naruto. This is all very speculative, of course, as it's not part of the original Naruto canon.
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.
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.
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.
He would likely become extremely powerful. His combat abilities would be enhanced significantly as the Rinnegan gives access to powerful jutsus. He could also gain new insights into the nature of chakra and the world around him.
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 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.
One possibility is that there was some sort of spiritual or chakra - based connection between Naruto and Nagato. Nagato, being the possessor of the Rinnegan, recognized Naruto's strong will and the fact that he was on a path to change the ninja world for the better. Through this connection, Nagato was able to transfer the power of the Rinnegan to Naruto. It could also be related to the nature of the Sage of Six Paths' power that was somehow involved in this transfer, as both Naruto and Nagato were related to the Sage in different ways.
In some fanfictions, Naruto might obtain the Kyuubi's bloodline through a special ritual. This could be a ceremony performed by a powerful shinobi or a sage who has the knowledge to transfer the essence of the Kyuubi's power into Naruto's genetic makeup, thus giving him the bloodline.
Well, in some fanfictions, Naruto might obtain the sword through a special bond or trial. Maybe Kyuubi decides that Naruto needs this weapon to face a new and powerful enemy. So it could be that Naruto proves his worth in some way, like showing great courage or a new level of control over his chakra, and as a result, Kyuubi gives him the sword.