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.
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.
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.
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.
In many Cornelia x Guilford fanfictions, sacrifice is a theme. They might have to sacrifice their own desires or safety for the sake of the other or for a greater good. This adds depth to their relationship and makes the story more engaging. Also, the theme of redemption can be present if one of the characters has made a mistake in the past and is trying to make up for it in the context of their relationship.
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.
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.