First, give the OC a clear backstory. For example, if they grew up in a small village, it could shape their values and skills. Then, define their goals. If they want to become a great ninja to protect their family, it will influence their actions. Also, give them unique traits like being able to sense emotions or having a photographic memory. These traits will make them stand out and be more interesting in the story with Madara.
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.
Power is a common theme. Madara is extremely powerful, so often the oc either has to match his power level in some way or is in awe of it. For example, in some fictions, the oc might be a powerful ninja herself and they engage in epic battles together.
One common theme is power imbalance. Since Madara is a very powerful character, often the OC has to deal with his strength and influence, which creates interesting dynamics in their relationship.
One common theme is friendship. Despite their differences, Madara, Hashirama, and the OC often form a unique friendship that transcends the normal boundaries in the Naruto universe. Another theme is power. The OC usually has some sort of power that either attracts or challenges Madara and Hashirama.
One common theme is redemption. Often the fem OC tries to redeem Madara from his darker paths. Another theme is love that defies all odds, like clan differences or Madara's past actions. Also, the exploration of power dynamics between them can be a theme, as Madara is a very powerful character.
A common theme is power. Often, the OC has some special power or ability that either rivals Madara's or complements it. This creates an interesting dynamic between them.
I really like 'Eternal Connection'. In this fanfic, Madara and Itachi are brought together in a time - travel scenario. It's a really interesting concept as they both have to deal with the knowledge of the future and past while also developing their relationship. It's full of action and some really touching moments.
Well, 'Three Hearts in a Shinobi World' is another great one. The author does an amazing job of creating a believable world where the OC, Madara, and Hashirama interact. The story is full of action, drama, and some really touching moments as the three characters form an unexpected bond that changes the course of the ninja world in the story.