One great 'Naruto and Madara Uchiha' fanfiction is 'The Redemption of Madara'. It offers a unique take on how Madara could have been redeemed in the Naruto universe. It has a great plot that keeps you hooked from start to finish.
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.
One popular theme is redemption. Many fanfictions explore the idea of Madara being redeemed through his interactions with Naruto. For example, Naruto's unwavering belief in the good in people might make Madara question his own actions and motives.
One of the most interesting aspects is the character transformation. Seeing Naruto, who was always full of hope and positive energy, take on Madara's dark and brooding persona. It's like a complete 180 - degree turn. Another aspect is how the other characters react. For example, Hinata might be completely lost, not knowing how to deal with this new Naruto. Also, the power - up Naruto would get as Madara. He would have access to the Sharingan and all of Madara's combat skills. It would be exciting to see how he uses them in battles, especially against opponents he used to face as Naruto.
If Naruto becomes Madara Uchiha in fanfiction, it could drastically change the plot. Firstly, the relationships would be turned upside down. Naruto's friends might see him as an enemy, especially Sasuke who has a complex relationship with the Uchiha legacy. The power dynamics in the ninja world would also shift. Naruto, with Madara's power and knowledge, could either rule with an iron fist or try to reshape the world in a more extreme way compared to his original goal of peace. And the Akatsuki's plans would likely be disrupted as Naruto, in Madara's form, might have different motives regarding the tailed beasts.
One idea could be Madara being transported to the RWBY world and initially being seen as a great threat. But then he starts to understand the unique nature of Dust and tries to use it in his own jutsu. Another plot could involve him training some of the RWBY characters, like Ruby, in his powerful Uchiha techniques, which would give them an edge against the Grimm.
Effective character development involves giving each character a goal. The OC could have a goal of finding her true place in the ninja world. Madara's goal of uniting the clans and Izuna's goal of protecting his family will then interact with the OC's goal. Show how they conflict and align. Also, give them flaws. The OC might be too trusting at times. Madara could be overly ambitious to the point of being blind to some things. Izuna might be too deferential to Madara. By having these flaws, they become more relatable and the relationships more complex and interesting.
One idea could be a love triangle where the OC is caught between Madara's grand ambitions and Izuna's more personal loyalty. The OC might be a healer who tries to keep the peace between the two brothers while falling in love with both in different ways. For example, Madara's strength and leadership might attract the OC on one level, while Izuna's kind heart and protectiveness draw her in another way.
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.
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.