Love and loyalty also pop up a lot. Highschool DxD has a lot of elements related to love and loyalty within its harem - like setting. When combined with Code Geass, fanfics may show how characters develop new relationships or test their existing loyalties. For instance, a character from Code Geass might find themselves in a situation where they have to choose between their loyalty to their original cause and the new relationships they form in the world of Highschool DxD.
One common theme is the idea of identity. In Code Geass, Lelouch has to hide his identity as Zero while in High School DxD, the main character has to deal with his new identity as a devil or whatever supernatural being he becomes. In the fanfics, this is often explored further, like when a character with a Geass has to hide it while also dealing with their new identity in the DxD world. Also, loyalty is a theme. In both series, there are groups and factions, and in fanfics, characters have to decide where their loyalties lie when these two universes collide.
One popular 'Code Geass and Highschool DxD fanfic' is 'Code DxD: Lelouch of the Gremory'. It combines the strategic elements from Code Geass with the supernatural and harem aspects of Highschool DxD in an interesting way.
Unique crossovers can make a fanfic stand out. For example, if it combines the magic and devil - related elements of Highschool DxD with the strategic and political aspects of Code Geass in an interesting way. Maybe the main character has to use Geass powers in the Highschool DxD world to deal with devils and fallen angels.
You can start by looking on fanfiction websites like FanFiction.net or Archive of Our Own. They usually have a large collection of fanfics for various shows, including Highschool DxD and Code Geass. Just use their search functions and filter for the relevant fandoms.
One common theme is Gino's loyalty. Many fanfics explore how his loyalty to the Britannian Empire or to certain characters affects his decisions. For example, how he balances his loyalty to the empire with his personal beliefs.
One common theme could be power struggles. Since Code Geass is about a world with complex power dynamics, Marrybell OC fanfics might explore how an original character fits into that, whether they are trying to gain power, resist it, or something in between.
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 common theme is power acquisition. Since 'Code Geass' is about special powers, self - insert characters often gain a unique Geass or some other extraordinary ability and then have to figure out how to use it in the complex world of the series.
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.