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.
One possible way is that during a battle, Naruto and Madara's chakra interact in an unexpected manner. Naruto's own vast chakra reserves, combined with his unique Uzumaki bloodline, might create a situation where his body is able to assimilate the Rinnegan when Madara's defenses are down. This could be due to Naruto's unwavering will and his ability to connect with others on a deep level, which allows him to accept and control the powerful Rinnegan.
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.
One possibility is through some sort of transplant or transfer. Maybe Madara, before his death, found a way to transfer his Rinnegan to Naruto. It could be a last - ditch effort to pass on his power to someone he saw potential in. Another way could be through some sort of ancient jutsu or curse that links their fates and causes the transfer of the Rinnegan.
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 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.
One implication is that Naruto becomes an even more powerful force in the ninja world. With the Rinnegan, he can access new abilities like the Six Paths techniques, which would make him nearly unbeatable in combat.
Naruto would probably win. He has an indomitable will and has always found ways to overcome powerful foes. His experience and the unique powers he might gain with the Rinnegan could give him an edge over Madara.
Naruto would likely gain the ability to use all six paths techniques. The Rinnegan allows the user to manipulate gravity, as seen when Madara used it. Naruto could use this to enhance his combat skills, for example, pulling enemies towards him or pushing them away forcefully. He could also use the Preta Path to absorb chakra, which would be a great addition to his already large chakra reserves.
He could combine the Rinnegan's ability to control gravity with his Rasengan, creating a super - powerful and hard - to - dodge attack.
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.
Power. Most fanfictions focus on the extreme power of Madara's Rinnegan. They explore how it can be used to dominate battles or change the Naruto world. Another common theme is mystery. Since the Rinnegan has many unexplored aspects in the original series, fanfictions often try to uncover its secrets.