One idea could be an OC who is a time - traveler and meets Uchiha Madara in his prime. The OC could have knowledge of future events and try to change the course of history, leading to many power struggles and moral dilemmas.
An OC who is a healer could be interesting. In a world full of battles and destruction, this OC tries to heal the wounds of the ninja world while interacting with Madara. Their different views on how to achieve peace, with Madara's more forceful approach and the OC's more peaceful one, could create a lot of tension and story development.
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.
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.
One idea could be a crossover with the Harry Potter universe. Madara Uchiha could be a powerful dark wizard from a far - off land who arrives at Hogwarts. He might be intrigued by the magic there and try to either conquer or learn from it. Another idea is a crossover with the DC universe. He could be pitted against Superman, and his Sharingan powers would make for an interesting battle against the Man of Steel. He might also form an alliance with Lex Luthor, thinking they have similar goals of power and domination.
One idea could be a time - travel plot where Sakura is sent back in time and meets Madara. She tries to change his path of destruction. Another could be a scenario where Madara is resurrected in Sakura's time and they have to team up against a new common enemy. Maybe a plot where Madara, in his search for power, discovers Sakura's unique medical and combat abilities and tries to use her for his own gain.
Well, a plot could involve Sakura and Madara being forced to team up against a new, powerful enemy that threatens the entire ninja world. Sakura initially distrusts Madara, but as they fight side by side, she begins to see a different side of him. Maybe there's a plot where Madara is cursed, and Sakura is the only one who can break the curse because of her unique chakra. As she tries to break the curse, they get to know each other better. Or perhaps in a parallel universe, Sakura and Madara are lovers from the start and they have to fight against the forces that want to tear them apart.
One idea could be that the OC is a foreign ninja who comes to the Hidden Leaf Village and Shisui is assigned to show her around. They start to bond over their different views on ninja life. Another could be that the OC has a special ability that Shisui needs to help him in a mission against a common enemy. And finally, perhaps the OC is a healer who saves Shisui's life during a battle, and he feels indebted to her which leads to a deeper connection.
One idea could be that the OC is a time - traveler from a future where Madara's actions had led to a desolate world. The OC tries to change Madara's path. Another could be the OC is a healer who stumbles upon a wounded Madara and nurses him back to health, leading to an unexpected bond. Or perhaps the OC has a unique power that Madara is intrigued by and he tries to use it for his own purposes at first but then develops real feelings for the OC.
I'd recommend 'Madara and the Unseen Ally'. In this fanfic, the OC has a unique power that catches Madara's attention. The plot unfolds as they face various challenges together, and the writer does an excellent job of portraying Madara's character while also giving the OC a distinct personality.
She could be a healer in the fanfiction. In the world of Naruto, healers are very important. Sasuke's sister OC could use her healing skills to help not only her friends but also enemies in need. This would create a lot of interesting situations where she has to make moral choices. For instance, should she heal a ninja from an enemy village who has information that could save her village? It would also put her in a unique position within the Uchiha clan as a valuable asset.
First, you need to understand Madara's character thoroughly. Know his motives, his powers, and his place in the Naruto universe. Then, create an OC that can interact meaningfully with him. For example, if your OC is a pacifist, how would they deal with Madara's more violent tendencies? Make sure to have a solid plot that ties in with the Naruto lore.