For the OC, start by defining their backstory. Where are they from? What experiences have shaped them? If they are from a lower - class area in Britannia, they might have a different perspective on the empire compared to Cornelia. Then, think about their goals. Are they seeking revenge, justice, or something else? As for Cornelia, explore her loyalty to Britannia deeper. Show her internal conflicts, like her doubts about some of Britannia's actions. Also, in their interactions, let their characters evolve. For example, if the OC challenges Cornelia's beliefs, she could start to question herself more, and the OC could learn to be more diplomatic from Cornelia.
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 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.
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.
Realistically, their relationship could start from a place of mutual enmity. Since they are on opposite sides in the original story, any development would require a significant shift in circumstances. For example, if they are both betrayed by someone within Britannia, they might find common ground in seeking revenge or justice.
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.
First, study the existing world of Code Geass. Know the politics, the social hierarchy, and the technology. For example, if your OC is a Britannian, they should act according to the values and norms of that society. Second, give your OC a clear motivation. It could be revenge, love, or a desire for power. And third, don't make them overpowered. Even if they have a special ability, it should have limitations.
One idea could be an OC who is a commoner but has a unique power that gets noticed by Lelouch. For example, the ability to manipulate sound waves which could be used for both stealth and attack. Another could be an OC from a foreign nation who has different moral and cultural values than the Britannians and Japanese, creating interesting conflicts. Or an OC who is a brilliant scientist working on a secret project that could change the balance of power in the Code Geass world.
Portraying Cornelia well means exploring her complex emotions. She's not just a cold military leader. She has feelings for her sister, for instance. So, you could write scenes where her softer side shows through, like when she's thinking about her family in private moments. Also, her pride in Britannia is a big part of her character. Illustrate how she upholds the honor of Britannia even in difficult situations. And don't forget to show her strategic mind at work during military campaigns.
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.