Well, one possible plot could be that Naruto grows up learning Earth - style jutsu from Onoki at a very young age. This gives him a different combat style compared to the original series. He might also inherit Onoki's no - nonsense attitude towards enemies.
Well, in such a fanfiction, it might start with Naruto being in a situation where he becomes Madara's pet. Maybe at first, Naruto is defiant but as time passes, he could start to understand Madara's motives in some strange way. Madara, on the other hand, might initially see Naruto as just a possession but gradually develop a sense of attachment. It could be a really strange and complex relationship that unfolds as the story progresses.
Madara would have to adapt to the different threats in RWBY, mainly the Grimm. In his world, he was used to fighting other shinobi and clans. The Grimm are a whole new type of enemy. However, his Sharingan would give him an advantage in detecting their movements. He might also try to find a way to use the Grimm's negative energy, similar to how he used negative emotions in his own world to fuel his power.
His personality might be more serious. Since Onoki is a no - nonsense leader, Naruto could pick up that trait. He might be less impulsive and more calculating in battles.
In a Naruto fanfiction where he has Madara's attitude, he could become more power - hungry. Madara was always seeking power to reshape the world according to his ideals. Naruto might start to view the tailed beasts not as friends but as tools for greater power. He could try to gather more chakra and perhaps even attempt to unlock new forms of power that are more in line with Madara's dark and ambitious nature, which would shock his comrades and enemies alike.
In some fanfictions, Naruto could be depicted as Madara's reincarnation. Fanfiction allows for a lot of creative liberties, so authors might come up with such an interesting concept to explore new storylines and character dynamics. However, in the original Naruto series, there is no such indication that Naruto is Madara's reincarnation.
I'm not sure about a specific 'naruto is madara fanfic' off the top of my head. However, if you search on large fanfic archives like FanFiction.net or Archive of Our Own (AO3), and use the tags 'Naruto' and 'Madara' along with something like 'admiration' or 'fan of', you're likely to find some relevant stories. These platforms have a wide range of stories written by fans from all over the world.
The plot could involve Naruto studying Madara's old techniques and strategies. He might be fascinated by Madara's ability to control the Nine - Tails in the past. So, Naruto starts to dig deeper into Madara's history and teachings, trying to incorporate some of his ideas into his own ninja way while still maintaining his core values.
In the fanfiction 'Training with the Legend', Madara starts training Naruto after realizing that Naruto has the willpower to face any obstacle. He imparts his combat strategies and survival skills. The story shows how Naruto adapts to Madara's strict training regime. Naruto's growth under Madara's tutelage is shown through his battles against Akatsuki members, where he uses the skills he learned from Madara to outwit and defeat them.
A fanfiction could explore what if the Gunbai was not just a weapon but a key to a different dimension. Naruto accidentally activates it and finds himself in a strange world filled with old - time ninja and powerful beasts. He has to find a way to get back to his own world while also dealing with the mysteries of this new place and the role the Gunbai plays there. This story could involve Naruto learning new jutsu and making new friends along the way.