One common theme is alternate universes. Writers like to explore what would happen if certain events in the original series had gone differently. For example, what if Lelouch had made different choices during his battles or negotiations.
Some dedicated anime forums might also be a good place to look for Code Geass Lelouch vi Britannia fanfics. For example, Anime-Planet has a community section where fans might share or recommend their favorite fan-written stories. Additionally, Reddit has subreddits related to Code Geass where you can ask for fanfic suggestions.
This fanfic idea presents a very interesting twist on the original Code Geass story. It would be exciting to see how the story unfolds with Kallen on the side of Britannia. Her skills and abilities could be used in different ways. For example, she could use her piloting skills to fight for Britannia against new threats, which would completely change the power dynamics in the Code Geass universe.
I'm not specifically familiar with 'Marianne vi Britannia' light novels. It could be a rather niche or new series. You might try searching for it in specialized light novel databases or forums to find out more about popular ones in this category.
You could start by looking in local bookstores that have a section dedicated to light novels. Some larger chain bookstores might carry them if they are widely available.
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.
She could start to reform Britannia from within. Kallen is a strong - willed character. If she joins Britannia, she may try to influence the system to be more just. For example, she could work towards improving the treatment of the Elevens within Britannia's hierarchical society.
A good Code Geass Lelouch fanfiction is 'Lelouch in a Parallel Realm'. In this story, Lelouch is transported to a parallel world where the power dynamics and politics are different. He has to adapt and use his wits in new ways. 'Code Geass: Lelouch's Alliances' is also great. It shows Lelouch forming unexpected alliances and how those relationships shape his journey. And 'The True Ending of Lelouch' is a fanfiction that tries to give a more satisfying and conclusive end to Lelouch's story.
The power dynamics. When Lelouch gives someone Geass, it completely changes the balance of power. For example, if he gives it to a character who was previously powerless, like a civilian, that character suddenly has the ability to influence others in a major way. This can lead to all kinds of new plotlines and character interactions.
C.C. is often a main character that interacts with Lelouch. Their relationship is complex and in the fanfic, they might continue to explore their connection, whether it's about the Geass or their personal feelings. Suzaku also could be a key character. He might have to come to terms with Lelouch being alive and they could either become allies again or have a more strained relationship.