In a Code Geass Akito the Exiled 5 years fanfic, perhaps Akito has become more powerful and is leading a resistance against the Britannian rule. He might have developed new Geass abilities or found new allies.
For Leila, she may have a more formal military look. Her uniform could be more decorated as she has climbed the ranks. Her hair might be styled differently to fit her new status. And in terms of the Britannian characters, they might have adapted to the new political situation. Some might have lost their pompous look if Britannia is in decline, while others might be more desperate to hold on to power and look more menacing.
One idea could be an OC who has a special power related to time manipulation. They could use this power to try and change the course of events in the Code Geass universe. For example, they might attempt to prevent certain battles or alliances from forming. Another plot idea could be an OC who is from a completely different world and gets caught up in the conflict of Akito the Exiled. They could bring in new technologies or ways of thinking that disrupt the status quo.
First, study the existing characters in Code Geass Akito the Exiled. Notice their personalities, their goals, and their relationships. Then, when creating your OC, make sure their traits are in line with the world. For example, if the world is very political, your OC should have some understanding or be involved in politics. Also, give your OC a backstory that makes sense in the context of the story. If there are certain factions, your OC could have a connection to one of them.
Since it's a fanfic, it could be really diverse in its content. It might be about the main characters from Code Geass having new adventures in the European part of the world during those five years. It could involve new mecha designs specific to that area, new cultures being introduced, and different ways that the characters interact with the local population. There could be a new antagonist or a group of antagonists that are causing trouble in this 'Euro' part of the Code Geass world, and the story could be about how the heroes deal with them over the course of five years.
I'm not sure which characters are the main ones in this particular fanfic. It could be Lelouch, as he is a central character in the original Code Geass series. Maybe it also includes C.C., who has a significant role in the story. But without reading it, it's hard to say for sure.
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.