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.
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.
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.
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.
Romance is a huge theme as well. It could be between a Shinto - inspired character and one of the main characters from Highschool DxD. For example, a love story between a kitsune (a common Shinto - related creature in fanfictions) and Issei. The cultural differences between Shinto and the other elements in the story can add an interesting dynamic to the romantic relationships.
One common theme is her growth and development. Since she is a strong character in the series, fanfictions often explore how she can become even more powerful or gain new abilities. Another theme is relationships. This could be her relationships with other characters from the show, like forming new friendships or even romantic relationships.
Another theme is her growth in power. Many fanfictions explore how Asia becomes stronger and how that affects her relationships with other characters like Issei. For example, in some stories, as she gains power, she also gains more confidence and is able to stand up for herself more. It's really interesting to see how her character develops in different ways in these fan - created stories.
One common theme is power. Ophis being a very powerful character, many fanfictions explore how her power is perceived and used in different situations within the high school setting.
One common theme is family loyalty. Since the Sitri family is an important part of the Highschool DxD universe, many fanfictions explore the bonds and loyalty within the family. For example, a Sitri character might go to great lengths to protect their family's honor or secrets.