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.
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.
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.
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 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 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 common theme is power and its corrupting influence. The male OC often has to deal with the allure and danger of power in the Code Geass world. Another theme is identity. Since the OC is original, there's a focus on how he discovers and defines himself within the context of the existing story.