One possible plot could be that after a dangerous battle where they almost lost each other, the tension and relief lead to a kiss. It could be a moment of realization for both of them about their deeper feelings.
Well, in this fanfiction plot, perhaps they get married as a strategic move at first. Lelouch thinks it will strengthen his position among the resistance forces with Kallen by his side as his wife. But as time goes on, they start to develop real feelings for each other. Kallen, who was initially skeptical, begins to see Lelouch in a new light and they build a relationship based on mutual understanding and respect. The story could then focus on how they navigate their new married life while still dealing with the political and military challenges around them.
It could signify a deepening of their bond. For Kallen, it might be the culmination of her admiration and loyalty towards Lelouch turning into a more romantic love. For Lelouch, it could be a moment of weakness where he allows himself to feel something genuine in the midst of his complex plans.
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.
A plot could involve Lelouch and Kallen being forced to work with an old enemy in order to stop a greater threat. At first, they are hesitant and distrustful, but as they face difficulties together, they realize that they can use this situation to their advantage. This situation also brings them closer as they share their strategies and secrets.
I'm not sure if there is a specific fanfiction about that. Fanfiction can cover a wide variety of scenarios, but this particular one might be quite rare or might not be in the mainstream. You could try searching on popular fanfiction websites like Fanfiction.net or Archive of Our Own.
One possible plot development could be that Kallen starts to question her own identity and purpose without Lelouch in a romantic sense. She might then throw herself into her work or other relationships. Meanwhile, Shirley and Lelouch could face challenges in their marriage, like political or social pressures, and Kallen might be reluctantly drawn back into their lives to help them.
In lemon fanfictions about Lelouch and Kallen, their relationship often takes on new and interesting dynamics. Sometimes, it might explore their unspoken feelings in the context of the story's unique plot twists. It could be a way to delve deeper into their characters, beyond what the original anime might have shown.
The kiss in the fanfiction might be a plot device to show a turning point in their relationship. It could be set in a time when all hope seems lost for their world. But through this kiss, they find the strength to fight together. It might be filled with passion, not just romantic passion but also the passion to save their world and the people they care about.
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.
One challenge could be political opposition. Since they come from different backgrounds, there may be groups who oppose their union for political reasons. Another is the public perception. They might have to deal with the opinions of the general public, which could be divided.