One possible fanfiction could be a post - war scenario. After the chaos and destruction in the Code Geass world, Lelouch and Kallen, having fought side by side, realize their deep connection. They start to rebuild the world together and gradually their friendship turns into love. Eventually, they get married in a small, private ceremony attended by their closest friends and allies. The fanfiction could focus on how they overcome the differences in their backgrounds and the challenges of leading a new world while being in love.
C.C. might view it with a sense of detachment at first. But then she might realize that Lelouch is becoming more attached to Kallen in a way that could potentially change their own relationship. She might start to feel a bit left out or even jealous, as she has been by Lelouch's side for a long time and has her own unique bond with him.
Another scenario could be a goodbye kiss. If Lelouch has to embark on a very dangerous mission where he may not return. Kallen, realizing she may never see him again, impulsively kisses him. It would be a bittersweet moment filled with unspoken words and the weight of their relationship and the situation at large.
One popular fanfiction might be 'A New World for Us' which explores their relationship in an alternate universe where the war ends differently. It delves deep into how they interact and support each other in rebuilding their world.
There's also 'The Unspoken Bond'. In this fanfiction, the author delves deep into the unspoken feelings between Kallen and Lelouch. It starts with small, subtle moments between them during the battles and gradually builds up to a more complex relationship. It's a great read for those who love to analyze the characters' emotions.
In many fanfictions, their relationship starts from comrades in arms. They are both fighting for the same cause against Britannia. As they face more and more difficult battles together, they start to understand each other better and a deeper bond forms.
In many fanfictions, their relationship often starts from mutual respect. Kallen respects Lelouch's strategic mind, and Lelouch admires Kallen's strength. For example, in some fanfics, they start to rely on each other more during difficult battles, which deepens their bond.
It could include their courtship. For example, Lelouch might start to show his softer side to Kallen, and she in turn would be more open about her feelings. They could go on secret dates in the rebuilt areas of the world, away from the prying eyes of the public. The fanfiction might also show how their friends react to their relationship. Some might be supportive, while others might be skeptical at first but then come around.
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.
You can start by looking on fanfiction platforms like FanFiction.net or Archive of Our Own. Just type in 'Code Geass Lelouch and CC kiss' in their search bars, and you'll likely find a bunch of relevant fanfictions.