Well, it might be that the granddaughter follows a trail of mysterious events that are somehow connected to Nunnaly and eventually leads her to him. Or perhaps she meets someone who knew Nunnaly and gets directions from them.
It's possible that the granddaughter discovers some forgotten technology or magic that helps her locate him. Maybe she has a vision or dream that gives her a hint about where to look.
Another way is to put her in different relationships. Let her interact with characters she didn't have much to do with in the original series. For example, if she were to meet some of the more radical anti - Britannian groups. Her reactions to their ideologies and methods could be used to show her growth and the evolution of her own beliefs. She could be conflicted between her loyalty to her family and her understanding of the oppressed people's cause.
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.
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.
In Code Geass fanfic, Nunnally is often a complex character. She is usually depicted as a symbol of innocence and hope. Writers might explore her relationship with Lelouch in different ways. For example, some fanfics might focus on how she would react if she knew all of Lelouch's secrets and plans. Her blindness also adds an interesting layer to her character, as it can be used to show her inner strength and vulnerability simultaneously.
The 'Code Geass 5 Year Fanfic' might expand on the original world - building by delving deeper into the politics of the different nations. For example, it could explore how Britannia's rule has changed over the five years, whether there are new rebellions brewing in different regions. It may also focus on the economic aspects. Maybe some regions are suffering more due to Britannia's policies, and this leads to new forms of resistance.
Fanfics often take on different genres as well. Some Code Geass Code Fanfic are action - packed adventures that take the characters on new quests outside of the original storyline. Others could be more of a mystery, where the characters have to solve a complex problem that was not part of the original plot. And then there are the ones that are more character - driven, focusing on the emotional growth and development of the characters.
One great fanfic in this category is 'The Geass Exchange'. It explores an alternate scenario where Lelouch gives his Geass to a different character, Suzaku. This creates a whole new dynamic as Suzaku, with his own set of beliefs and moral code, has to grapple with the power of Geass. The story delves deep into how the Geass affects his relationships, not just with Lelouch, but also with other characters in the Code Geass universe like Euphemia. It's a really interesting take on the concept of Geass transfer and how it can change the course of events.
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.
The fanfic might also focus on the themes of rebellion and sacrifice. The 'Black Rebellion' aspect can be expanded upon, like how the resistance fighters adapt to new threats. They could be facing a new type of weapon or a more brutal Britannian general. This would show their determination and the lengths they are willing to go to for freedom, similar to what was shown in the original series but with new twists.
One great 'Code Geass' fanfic focusing on characters could be one that delves into the backstory of Lelouch. For example, it might explore his life before the events of the series, his relationship with his sister Nunnally in more detail. Maybe it shows how he developed his strategic mind even at a young age. This kind of fanfic can add depth to the character and make the readers understand his motives better in the original series.