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.
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.
A good Naruto Rinnegan Madara fanfic should first of all stay true to the character. Madara is a powerful and complex character, and his Rinnegan abilities should be portrayed accurately. For example, his ability to control the tailed beasts using the Rinnegan should be shown in a way that makes sense within the Naruto universe.
I remember a fanfic that focuses on Madara's past and how he obtained the Rinnegan. It's really detailed and gives a whole new backstory to his character. It also shows how his Rinnegan powers evolved over time. This fanfic has great character development for Madara and also includes some interesting side characters that interact with him in different ways related to his Rinnegan abilities.
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.
One common theme is power exploration. Writers often delve into how Madara's Rinnegan powers could be used in different scenarios, like battles against other powerful foes. Another theme is redemption. Some fanfictions might explore if Madara could be redeemed with his newfound power. And of course, there are also stories about his past and how he obtained the Rinnegan in the first place.
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.
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.
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.