In some fanfictions, Naruto might awaken the Rinnegan through a powerful jutsu overload. For example, if he combines a super - charged Rasengan with some ancient and forbidden chakra - based techniques. This could potentially trigger a genetic or chakra - system shift that results in the awakening of 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.
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.
In some fanfictions, Naruto might awaken his Rinnegan through a powerful and ancient jutsu. For example, he could come across a hidden scroll that contains the knowledge and power to unlock this dojutsu. Maybe a mysterious sage or an old shinobi reveals the secret to him, guiding him through a series of trials that eventually lead to the awakening of the Rinnegan.
One potential antagonist could be Orochimaru. He is always after powerful jutsus and the power of the Rinnegan would be extremely appealing to him. He might try to either steal Naruto's newly awakened Rinnegan or prevent Naruto from fully mastering it so that he can be the most powerful one in the ninja world.
It might be related to a connection with some powerful entity in the Wave arc. For example, if there's a secret shinobi from the past whose power is tied to the Rinnegan and Naruto somehow forms a bond with that power source, it could lead to the awakening. Also, the intense emotions and challenges he faces in the Wave arc might be a catalyst, pushing his chakra to a new level and awakening the Rinnegan.
One possible interesting plot could be Naruto discovers a hidden ancient scroll that details the secrets of awakening the Rinnegan. He then embarks on a dangerous journey filled with various trials and battles against powerful enemies who also want the power of the Rinnegan for themselves.
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.