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.
Perhaps an OC who was raised by a group of forest spirits. The spirits taught the OC how to use Wood Style as a means of protecting the forest. This OC would have a very different view of the world compared to normal ninjas. Their Wood Style would be more in tune with the natural balance, and they might be hesitant to use it for violent purposes, only doing so when the forest or its inhabitants are in great danger.
The Wood Style OC could be a rival to Naruto. Since Naruto has the power of the Nine - Tails, the OC with Wood Style could see Naruto as a threat to their own status as a powerful ninja. This would lead to intense battles and a begrudging respect as they learn more about each other's abilities and motives.