One idea could be that the reincarnated Madara retains some of his past memories but is in a completely different world. So he has to adapt to new cultures and powers while still having his old ambitions deep inside. Maybe he tries to find a way to bring back the old ninja ways in this new world.
A third plot idea is that the reincarnation process is incomplete, and Madara is split into two personalities. One is his old, power - hungry self, and the other is a more empathetic side that has been influenced by the new world. He struggles with these two sides of himself as he gets involved in a conflict that could decide the fate of both the new world he is in and the remnants of his old one.
Another plot could involve the reincarnated Madara being a student in a modern - day ninja academy. He has to hide his true identity as he discovers that the world has changed a great deal. He could form unexpected friendships and rivalries as he tries to rise to power in this new setting. He might also face a new, powerful enemy that he has to defeat using his old skills and new knowledge.
The character could start off with all of Madara's arrogance and then gradually learn humility as he experiences new things in his reincarnated form. For example, if he is reincarnated into a weak body, he has to rely on others at first, which is a new concept for him.
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.
A cool plot idea is an SI OC who is from a world where music has power. In Naruto, they discover that their musical abilities can either soothe or disrupt chakra. For example, they might be able to play a tune that calms down a rampaging bijuu. Another could be an OC that has the ability to see the past of a place just by touching the ground. This could lead to uncovering long - lost secrets about the founding of the ninja villages. Or perhaps an OC who can transform into different elements, not just the basic ones in the Naruto world, like turning into a magnetic field to disrupt enemy attacks.
Another plot could revolve around an OC who is from a different time or dimension and has knowledge of advanced technology. They could use this knowledge to try and create new defenses against the Titans. Maybe they develop a weapon that doesn't require blades and 3D maneuver gear but is more effective in taking down Titans.
One idea could be an OC who has the power to manipulate time in a limited way. They could use this power to try and prevent certain tragedies in the Code Geass world, like the fall of certain regions. Another plot could involve an OC who is a master strategist from a different universe and has to adapt their skills to the complex politics of Code Geass. A third idea might be an OC who can communicate with machines, which could lead to some really unique interactions with the mechs in the series.
A great plot idea could be an SI OC who has the ability to communicate with animals on a deep level, like a reverse Inuzuka. This OC could use the animals' knowledge of the land and hidden passages to gain an advantage. They could also form alliances with powerful animal summons, like the giant toads or snakes, but in a way that is different from the traditional summoners. This would create a whole new dynamic in the story as they navigate the politics and battles of the Naruto world.
Perhaps the fanfiction could center on the unborn child's potential power. Madara might worry about the child being used as a weapon or a tool by his enemies. There could also be a side plot about the reactions of the other ninja clans when they find out about Madara's pregnancy, and how it might change the power dynamics in the ninja world.
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.
The student could be from a different village or clan and is sent undercover to learn from Madara. As the student gets closer to Madara, they start to understand his motives and the true nature of power in the ninja world. They might then have to decide whether to betray their own village or side with Madara, leading to a complex web of relationships and plot twists.
One idea could be an OC who is a brilliant detective in Gotham, trying to solve cases alongside Batman but often having a different approach. For example, the OC might focus more on the psychological aspects of the criminals, while Batman is more into the physical evidence. Another idea is an OC who has a special power, like the ability to communicate with animals, which helps Batman in his fight against villains who use animals in their crimes. A third idea could be an OC who is from a different dimension and has a different set of values, creating interesting conflicts and collaborations with Batman as they try to understand each other.