One idea could be a story where Cornelia and Guilford have to go undercover in an enemy territory. They would have to rely on each other more than ever, facing challenges that test their loyalty and their relationship. Maybe they discover a secret plot that could change the course of the war in Code Geass.
Another interesting plot could be a 'what - if' scenario. What if Cornelia was injured and Guilford had to take over her duties? He would be in a difficult position, having to make decisions that Cornelia would normally make. This could lead to him understanding her better and also to some internal conflicts within the military structure of Code Geass.
One plot idea could be exploring Cornelia's relationship with her subordinates in a more in - depth way. Maybe she has to lead a risky mission and how she overcomes the challenges within the team. Another could be a story set in an alternate universe where Cornelia has to make different choices that change the fate of the Britannian Empire.
It can start with showing their professional relationship. They are comrades in Code Geass, so the fanfic could depict how they respect each other's skills in battle and in strategy. Then, gradually introduce personal moments, like when Guilford saves Cornelia from a dangerous situation and she starts to see him in a different light.
A fanfic about Cornelia could be a character study. It could delve into her past, like her upbringing in the imperial family and how that shaped her into the strong - willed person she is. Maybe it could also show her softer side that she doesn't often display in the anime.
One interesting plot idea could be that the OC is a hidden rebel within the Britannian forces. They initially cross paths with Cornelia during a battle, and the OC's unique abilities catch Cornelia's attention. As the story progresses, they start to understand each other's motives better, despite being on opposite sides. Maybe the OC has a secret power related to mind - reading which helps them anticipate Cornelia's moves, and this leads to a series of tense encounters that gradually turn into a strange sort of respect and eventually something more.
A possible plot could involve Lelouch losing his Geass powers and being captured by Cornelia. Instead of punishing him immediately, she decides to study him to find out more about the Geass. During this time, they engage in many deep conversations. Lelouch tries to convince Cornelia of the wrongdoings of Britannia's system, and Cornelia starts to question her own beliefs. This interaction could lead to a romantic relationship as they both grow and change through their exchanges.
A fanfic could center around Cornelia's relationship with her sister, Euphemia. It could show their sisterly bond, their differences, and how they support or oppose each other in the complex world of Code Geass.
Maybe the Black Prince discovers a new form of Geass power that no one else has ever seen. He then has to figure out how to control it while also hiding it from those who would seek to use it for their own evil purposes. This could involve a journey of self - discovery as he battles his inner demons as well.
One idea could be exploring Suzaku's internal struggle in a different light. Maybe he discovers a new power within himself that makes him question his loyalty to Britannia even more. For example, he could have a dream where he sees an alternate future of Japan if he fully embraced the Japanese side from the start.
Perhaps Lelouch from Code Geass ends up in the Naruto world. He could use his intelligence and strategic skills to help the shinobi against a common enemy. He might form an alliance with Naruto, and together they come up with plans to take on powerful foes like Akatsuki. Lelouch could also use his Geass to deceive the enemies and gain an upper hand in battles.
One idea could be a story where Nunnaly discovers her hidden power in the Code Geass world. Maybe she has a special ability that was dormant all this time and it starts to surface when she is in a life - threatening situation. This could change the power dynamics in the story and her relationship with other characters.