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.
Maybe an orphan from a non - Uchiha clan who has a special ability that Madara finds interesting. This character would be malleable and eager to learn from Madara, as they have no other real family or guidance. They could bring a fresh perspective to Madara's teachings.
In this kind of fanfic, the author may describe Naruto's resemblance to Madara in terms of their auras. Naruto could have an aura that is as intimidating as Madara's. For example, when he enters a room, people might feel the same kind of pressure as they would when Madara was around. And perhaps Naruto starts to have some facial expressions that are similar to Madara's, like a certain smirk or a serious frown.
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.
Well, in hinata and madara fanfiction, the theme of discovery is quite common. Hinata discovers new aspects of the world or herself through her encounters with Madara. Another theme is the clash of ideals. Hinata's pacifist ideals might clash with Madara's more militant ones, and how they resolve this difference is often explored. There's also the theme of forbidden love in some cases. Given their different standings in the Naruto universe, a relationship between them can be seen as forbidden, and fanfictions often play with this idea in interesting ways.
Fanfiction about Madara's memories can also focus on his relationship with Hashirama. We often see in these stories how their friendship was initially pure but then got torn apart by the political situation. This can give us new perspectives on the whole ninja world's history and how the two most powerful shinobi of their time had different views on peace.
One common theme is their friendship. Despite being on opposite sides at times, many fanfictions explore the deep - seated bond they had. Another theme is redemption. Madara, especially, is often the subject of redemption arcs in fanfiction. Also, the theme of sacrifice for the greater good is seen, as both characters in the original story were willing to make sacrifices for their respective villages.
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.