In such a fanfiction, Naruto and Sasuke might win. They are both extremely powerful and their combined strength, like Naruto's Rasengan and Sasuke's Chidori, could overpower Madara.
Well, one possible outcome could be that Naruto and Sasuke manage to defeat Madara through their combined strength and newfound powers. They might use their signature jutsus in a coordinated way that Madara can't counter.
Sasuke is often a popular character in these fanfictions. He might play a role in helping Naruto against Madara or have his own motives during their battle. Kakashi can also be a prominent figure, perhaps using his Sharingan in a crucial moment to assist Naruto. Another popular character could be Hashirama, as his connection to Madara is a key part of the original story and can be further explored in fanfiction.
One interesting plotline could be Naruto discovers a new form that gives him an edge over Madara. For example, a form that allows him to nullify Madara's Sharingan abilities. Another could be Madara having a change of heart due to Naruto's unwavering belief in him. Maybe he starts to question his own motives for destruction.
Sasuke could also be the one. With his Sharingan and Rinnegan, he has unique abilities. He might use his Amaterasu in a very unexpected way, like creating a large - scale trap for Madara using the black flames. His intelligence and cold - blooded nature in battle make him capable of making a decisive move.
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.
They would likely have a complex relationship. At first, they might be wary of Madara as he has been their enemy in the past. But as they train under him, they would start to respect his skills and knowledge. Naruto might try to understand Madara's motives and see if there's any good in him. Sasuke, having an Uchiha connection, might feel a stronger sense of kinship but still be cautious. Madara, for his part, would probably see them as tools at first but gradually develop a strange sort of respect for their growth.
It really depends on the writer's preference. Some fanfictions might have Naruto winning because of his unwavering belief in friendship and his ability to draw on the power of the nine - tails. Others might have Sasuke winning due to his strategic mind and mastery of the sharingan.
It's not clearly stated in the original Naruto series that Konoha as a whole watches Naruto vs Sasuke fanfiction. Fanfiction is a creation of the fans outside the official story. However, it would be an interesting concept to think about. Some characters might be amused or even inspired if they were to read such fan - created stories, but there's no in - canon evidence for it.