In the fanfiction, it could be a result of a genetic mutation. Naruto has always had unique genetics due to being the Jinchuriki of the Nine - Tails. One day, due to some external factors like exposure to a rare type of chakra radiation, his genetic makeup changes, and he is born with the blue Rinnegan. This then leads to a whole new set of challenges and adventures for him as he has to deal with this unexpected power.
He could obtain it through a powerful curse. For instance, a long - forgotten enemy places a curse on Naruto that not only gives him the blue Rinnegan but also sets him on a path to either destroy the world or save it depending on how he uses the power.
Maybe there's a special chakra - infused object. Naruto discovers an ancient relic during one of his adventures. When he touches it, the object transfers its energy into him, and as a result, he gains the blue Rinnegan. This new power then becomes a key element in the story as he has to figure out how to control it.
He could obtain it through a unique chakra infusion. Maybe during a battle with a powerful enemy who has some connection to an ancient chakra source, Naruto is accidentally exposed to a golden - hued chakra that fuses with his own and transforms his eyes into the Golden Rinnegan.
It could be that in the fanfiction, Naruto is exposed to a special type of chakra or energy source. This energy source could be related to an ancient artifact or a long - lost power. As Naruto has always been at the center of extraordinary events, his encounter with this special chakra could be the catalyst for him to obtain the True Rinnegan. It might also be that his previous battles and experiences have slowly been building up to this moment, and this final push of some external or internal factor causes the True Rinnegan to emerge.
Perhaps he could obtain the Rinnegan through some sort of ancient ritual or a gift from a powerful entity. And to become Hokage, he would need to prove himself worthy through his deeds and leadership skills, like protecting the village from a major threat.
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 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 a fanfiction, Naruto might obtain the Rinnegan and Sharingan through a unique encounter. For example, he could stumble upon a dying Uchiha who transfers his Sharingan to Naruto in a moment of desperation. As for the Rinnegan, it could be related to his connection with the Sage of Six Paths. Perhaps the Sage's spirit, seeing the great need in the ninja world, bestows the Rinnegan upon Naruto. It could also be a result of some sort of genetic experimentation gone wrong but in a way that gives him these powerful eyes instead of harming him.
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 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.
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.