Love and attraction are very common themes. The idea of the reader being drawn to Madara's strength and mystery. Also, the theme of trust. Given Madara's complex nature, the fanfictions often deal with the reader building trust with him. Additionally, there's the theme of adventure. The reader and Madara might go on adventures together in the Naruto - like world, facing various challenges and enemies.
One common theme is the power dynamic. Madara is a very powerful character, so exploring how the reader interacts with that power is often seen. Another is redemption. The reader might be a character who tries to bring out the good in Madara. Also, love across different worlds or time periods can be a theme, especially if the fanfiction has an AU setting.
Love and relationships are another set of common themes. Fanfiction writers often explore how female Madara's relationships, whether it's with her clan members, friends, or rivals, are different from the male version. Her romantic relationships, if any, are also explored in a new context, like her potential relationship with Hashirama taking on a different dynamic due to her being female.
One of the main themes is revenge. In many fanfictions, Madara's actions are driven by a desire for revenge against those who have wronged the Uchiha clan. Also, the theme of family honor comes up. Madara is often portrayed as trying to uphold the honor of the Uchiha name during the chaos of the warring clans.
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.
One common theme is power dynamics. Madara is a very powerful character, so in the harem, there are often issues of how his power affects his relationships with the other members. Another theme is cultural clash. Since it's a crossover, characters from different backgrounds come together, which can lead to interesting cultural exchanges within the harem.
Often, Naruto might be paired with Madara in some sense. Their opposing views on power and peace could create interesting interactions in the fanfiction.
I would recommend 'Madara Uchiha x Reader: The Bond'. It shows a unique bond between the reader and Madara that develops over time. The story has great character development for both the reader character and Madara. 'Love in the Time of Shinobi' is also a good fanfiction. It's set during a time of great turmoil in the Naruto world and shows how the relationship between the reader and Madara blossoms despite all the chaos. 'The Uchiha's Heart' is a fanfiction that focuses on how Madara opens up his heart to the reader, which is a really fascinating concept.
One common theme is power. Since Madara was extremely powerful, fanfictions often explore how Naruto deals with this new - found power if he is related to Madara. Another theme is identity. Naruto has to figure out who he is now that he has this connection to Madara Uchiha.
Romance is also a popular theme. Writers might pair female Madara with other characters from the Naruto universe, like Hashirama. The relationship between them could be explored in a new light, with the gender - swap adding a different dynamic to their interactions.
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.