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 a 'naruto madara's apprentice fanfiction', the plot could be that the apprentice is from a small and forgotten clan. Madara sees the clan's ancient and unique abilities in the apprentice. He trains the apprentice not only in combat but also in strategy. The apprentice then has to deal with the fact that they are associated with Madara, who is seen as a villain by many. This causes the apprentice to have a complex relationship with other ninjas and also forces them to question their loyalty to Madara as they experience different aspects of the ninja world.
Maybe Valerie could be the apprentice. She has her own experience with fighting ghosts, so she would already have some skills. But as an apprentice to Danny, she could learn new techniques and ways of dealing with ghosts. Also, the dynamic between her and Danny could be really interesting, as they've had their share of conflicts in the past. It would be cool to see them work together in a more positive way. And there's also the potential for her to learn about Danny's secret identity in a new and different way as his apprentice.
A human with a mysterious past, like a former street urchin from Coruscant who has a natural affinity for the Force. This character would start off very naive about the ways of the Sith but quickly learn under Vader's harsh training. Their journey from a nobody to a powerful apprentice could be full of twists and turns.
One suitable character could be a healer's daughter. She has grown up seeing the power of natural remedies and has a natural inclination towards magic. She is kind - hearted and intelligent, which would make her a great apprentice for Merlin. Also, a young noble who has been shunned by his family for his strange dreams (which are actually premonitions due to his budding magic) could be an interesting choice. Merlin would see the potential in him and take him as an apprentice.
One good candidate could be an original character. For example, a powerful kunoichi from a small, peaceful clan. She could bring a different perspective to Madara's life. Another could be a character from another ninja village, like a Kiri ninja who was sent as a diplomat and ended up falling for Madara.
A young ninja with a lot of potential but also some major flaws could be a great main character. For example, a ninja who is very powerful but lacks self - control. The sennin could then focus on teaching them not just ninja skills but also self - discipline.
It might be one of the existing minor characters from the Naruto series who finally gets a chance to shine as Kakashi's apprentice. For example, that one ninja from the background who always admired Kakashi from afar and now has the opportunity to be his student.
In this fanfiction, Naruto's character development could be seen as a counterpoint to Madara's. Naruto is all about hope, friendship, and protecting the village. Madara has been through a lot of pain and betrayal which led him to a path of destruction. If Naruto is like Madara in this fanfiction, his development might involve a struggle between his own beliefs and the allure of Madara - like power and ruthlessness. However, Naruto would probably still find a way to use any new - found power in a more benevolent way compared to Madara.
It could be a young, inexperienced mage who has always admired Natsu from afar. Maybe someone with a hidden power that Natsu senses and decides to take under his wing.
Sasuke could be a main character as well. Given his own experiences of being an outcast and having a thirst for power at times, he could relate to Madara in some ways. They might form an unexpected alliance or have a rivalry based on their shared experiences of feeling neglected or misunderstood in the ninja world.