Well, often the theme of isolation is present. Lelouch was a very lonely character in many ways. In a suicide fanfic, this isolation could be exacerbated. He might feel that no one truly understands him or his motives. It could also explore the theme of power and its consequences. Lelouch had great power with his Geass, and his suicide could be a way of escaping the burden of that power. Maybe he realizes that the power has corrupted him in some ways or led to too much destruction, and death is his way out. And then there's the theme of legacy. His suicide could be seen as a way to leave a certain kind of legacy, whether it's a martyr - like one or a cautionary tale about the pursuit of power.
One more theme that pops up a lot is the exploration of 'what - if' scenarios. Fanfic writers like to play with the idea of what would have happened if certain events in the series had been different regarding Lelouch and Shirley. For instance, what if Shirley had found out about Lelouch's true identity earlier? This leads to different relationship dynamics and storylines in the fanfics, which is really interesting to read about.
One common theme is the exploration of gender roles. Since Lelouch is female in these fanfics, it often challenges the traditional male - dominated power dynamics in the original Code Geass world. For instance, a female Lelouch might have to use different strategies to gain power and respect.
One common theme is redemption. Since Lelouch has done many morally ambiguous things in the original series, some fanfics focus on him trying to make up for his past actions. Another theme is love and relationships, such as his relationship with C.C. or his sister Nunnally.
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.
One common theme is the exploration of their complex relationship. Given their unique personalities and the circumstances in the Code Geass world, fanfics often focus on how they understand and support each other. Another theme is the idea of power and sacrifice. Since both Lelouch and CC are tied to power in different ways, many fanfics will play with the concept of what they are willing to sacrifice for each other or for their goals.
They often show his internal conflict between his goals and his morality. Lelouch wants to change the world, but the methods he uses are often morally ambiguous. In a suicide fanfic, this struggle might reach a breaking point. He could be constantly thinking about all the wrongs he has committed in the name of the greater good. For example, he might be haunted by the memories of the people he has killed or manipulated. His Geass power also adds to his inner struggle. He may feel that the power has made him into a monster, and he can't live with that anymore, so he contemplates suicide as a way to end his inner turmoil.
Love and loyalty are common themes. The fanfictions often explore the deep bond between CC and Lelouch.
Power dynamics is also a theme. Euphemia was a princess and Lelouch was a revolutionary in a way. Fanfictions may explore how their relationship affects the power balance in the world of Code Geass. It could be about them using their combined influence for good or the struggles they face because of their different positions of power. And also, the theme of sacrifice might be present, as they both had to make sacrifices in the original story, and fanfictions can expand on that concept in relation to their relationship.
The themes of sacrifice and redemption are quite common. Rolo, in particular, has actions that could be seen as in need of redemption, and Lelouch's own goals often require sacrifice. Fanfics may focus on how Rolo tries to redeem himself in Lelouch's eyes or how Lelouch sacrifices for Rolo. Additionally, the theme of power and control is present. Lelouch has the Geass power, and how he uses it in relation to Rolo and their relationship can be a central theme in fanfics.
I'm not sure specifically as it's a fanfic. It could be about Lelouch from Code Geass in some sort of capsule - related situation. Maybe it's a unique take on his character, like he's trapped in a capsule and has to figure out a way to escape or use his Geass powers in a confined space. It could also involve some new plotlines with other characters coming to his aid or trying to stop him.