Often, Naruto might be paired with Madara in some sense. Their opposing views on power and peace could create interesting interactions in the fanfiction.
No. Naruto has his own unique story and character development that is distinct from simply being a fanfiction of Uchiha Madara. Naruto is about his journey to become Hokage, his relationships with his friends like Sasuke, Sakura, etc. His goal is to bring peace to the ninja world in his own way, through understanding and friendship. While Madara is an important part of the Naruto lore, Naruto's story is not centered around being a fanfiction of Madara. He is his own hero with his own dreams and challenges that are not directly related to being a fan of Madara.
Yes, there might be. The world of fanfiction is vast and full of crossovers. Fans often like to combine different fictional universes, so it's very possible that someone has created a story where Harry Potter is a fan of Madara Uchiha. You can search on various fanfiction platforms to find out.
In a Naruto and High School DxD fanfiction, there could be a scenario where the protagonists of both stories meet in a neutral dimension. Issei from High School DxD might be intrigued by Naruto's ninja skills and determination. Naruto, in turn, could be curious about Issei's Sacred Gear. They could start training together, sharing their knowledge and skills, and ultimately have to face a common enemy that threatens both their worlds.
Fanfiction.net is a great place to start. It has a large collection of 'madara uchiha crossover fanfiction' stories. You can use the search function and filter by the relevant tags to find exactly what you're looking for.
Another possibility is that Madara is a transfer student who is initially aloof and mysterious. Sakura, being curious, tries to befriend him. As time passes, she discovers that Madara has a troubled past. Sakura, with her kind heart, tries to help him. This could lead to a deep emotional bond between them, different from the typical high school relationships in the Naruto universe.
Well, if Naruto were Madara's brother in a fanfiction, it would completely change the Naruto universe. Their power levels combined could make them an unbeatable force. Naruto's positive attitude might have influenced Madara to not go down the dark path he did in the original story. It could also lead to different alliances and battles, with Naruto perhaps trying to reason with Madara and change his view of the ninja world.