Power. Since it involves dragon gods, a common theme is the acquisition and control of great power. Characters often struggle with newfound dragon god - like abilities.
Another common theme is identity. In these fanfics, the characters with dragon god elements often have to figure out who they are in relation to the normal DxD world. Are they a threat? A savior? It's all about self - discovery. For example, in many stories, the character might initially be shocked by their dragon god heritage and then have to come to terms with it.
Power - up is a common theme. Since Issei becomes a Dragon God, the fanfictions often explore his new powers and how he uses them to overcome challenges. Another theme is relationships. His relationships with other characters, like Rias and his friends, might change due to his new status.
One common theme is power. Since both Dragon Ball and Highschool DxD have characters with different levels of power, fanfictions often explore how these powers interact. Another theme is friendship. Characters from both series form new friendships or rivalries in the crossovers.
A common theme is power. The dragon slayer often has to deal with gaining and controlling their power in the Highschool DxD universe. Another theme is friendship, as they interact with the characters from the original series.
Power growth is a common theme. Fans often like to explore how the White Dragon Emperor can become even more powerful and overcome new challenges.
Power - up is a common theme. Writers often explore how the white dragon gains new abilities and becomes stronger in the Highschool DxD setting.
A good plot idea could be that the OC Dragon God has to hide their true identity in the high school. They are there to find a certain magical item that is hidden in the school. As they search, they get involved with the main characters and have to deal with the normal high school drama as well as the supernatural threats that come their way.
One of the most exciting aspects of Highschool DxD OC Dragon God fanfiction is the creation of unique characters. Writers can design their own original characters (OCs) with dragon god powers, which adds a whole new layer of intrigue to the story. For example, an OC dragon god could have the power to manipulate time, leading to some really cool plot twists.
Themes like adventure are quite common. Picture a character from DBZ exploring the strange and exciting world of Highschool DxD, or vice versa. There's also the theme of competition. Given that both universes have strong characters, fanfictions often set up scenarios where they compete against each other. Love is also a theme in some cases, where characters from the two different universes fall in love and have to deal with the differences between their worlds.
One common theme is loss. It could be the loss of a loved one, like a friend or a family member in the context of the High School DxD world.
One common theme is power - up. Siegfried is often depicted as getting stronger or unlocking new abilities. This ties in with the overall power - fantasy aspect of the Highschool DxD universe.