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.
Another theme is redemption. Sometimes in these fanfics, Madara has a chance at redemption through his fight with Naruto. Naruto's belief in people's goodness can influence the outcome in such scenarios.
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.
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.
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.
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.
In certain 'guy vs naruto fanfiction', the guy could be a magic user from a world without ninjas. His magic might work in a completely different way compared to Naruto's chakra - based powers. For example, he could use elemental magic that is not the same as Naruto's use of the five elements. Their fight would be a clash of two different power systems. Also, the guy might have a different moral code that affects how he fights Naruto. He could be more ruthless or more cautious, depending on his background, which adds another layer of complexity to the story.
There's also 'Naruto's Ascendance Against Madara'. In this fanfic, Naruto discovers a new form that gives him an edge in the fight against Madara. It's exciting because it shows a different side of Naruto's potential that wasn't fully explored in the original series.