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.
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.
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.
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.
I think young Sasuke would have a very intense reaction. He was so consumed by his hatred for Itachi at that young age. A Sasuke vs Itachi fanfiction would be like a trigger for him. He would be eager to see how the fictional fight is portrayed. If Sasuke in the fanfiction is shown as strong, it might fuel his own ego and make him strive even harder to become powerful. However, if the fanfiction shows Itachi as superior, it could enrage him. He might feel that it undermines his own goal of defeating Itachi and he might even want to find the writer to correct their 'wrong' view.
One common theme is redemption. Sasuke might see Madara's past and try to change his future path towards destruction. Another theme is the exploration of family heritage as they are both Uchihas. They may discover secrets about their clan together through time travel.
For Naruto, being trained by Madara could mean he learns how to counter Uchiha - specific techniques more effectively. His physical strength might also increase due to the intense training. Sasuke would be able to better harness his hatred and turn it into a more focused power source. Madara's teachings could expose them to ancient jutsu and knowledge that would give them an edge over their enemies. Their teamwork might also be affected, as they would both be influenced by Madara's way of thinking.
Finding a good 'madara x hinata x sasuke fanfic' depends on your personal taste. Some people like fics that focus more on the emotional relationships between the characters, while others might prefer action - packed stories. You can start by searching for these character names in combination on different fanfic sites and then explore the ones that seem the most appealing based on their summaries.
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.