One common theme is power. Since the OC is a god, there's often a focus on their extraordinary powers and how they use them in the high school setting of DxD.
A prevalent theme is also the disruption of the status quo. The presence of an OC god in the DxD high school is bound to shake things up. It could lead to changes in the power dynamics among different factions in the school, like the devils, angels, and fallen angels. And often, there's a theme of the OC god having to adapt to the rules and culture of the DxD high school world, which is different from what they might be used to in their god - like existence.
Another theme is relationships. The OC god might form various relationships with the existing characters in the DxD high school world. It could be friendships, rivalries, or romantic relationships. For example, they might have a complex relationship with the main characters like Issei. Also, the theme of self - discovery can be present. As the OC god navigates the high school environment, they may learn more about themselves and their place in this fictional universe.
One common theme is power growth. The OC usually starts with some level of power or potential and then grows stronger as the story progresses, often in relation to impressing or protecting the members of the harem. Another theme is the exploration of relationships. Since it's a harem fic, there are multiple love interests, so the OC has to navigate through different types of relationships, like friendship - turned - love or instant attraction.
Transformation is also a popular theme. Given ACNologia's nature, fanfictions may involve him undergoing some sort of transformation, either physically or in his personality. This can be due to the influence of the High School DxD environment or his interactions with the characters. And this transformation can then drive the plot forward in interesting ways.
Friendship is also a big theme. Characters from both universes form new friendships and alliances. For example, Naruto's idea of comradeship might influence the characters in High School DxD and vice versa. They learn from each other's values and work together to face challenges, whether it's school - related in the High School DxD setting or something more global like in Naruto's world.
One common theme is the exploration of relationships. Given the nature of the original series, many fanfictions focus on the romantic or friendship relationships between Uri and other characters.
One common theme is power growth. Many fanfictions focus on the main character getting stronger in the High School DxD universe, whether it's through new powers or better control of existing ones.
One common theme is Asia's growth. Since she starts as a rather timid character in the original, fanfictions often focus on her becoming stronger. Another theme is her relationships, like her friendship with the main characters.
One common theme is power growth. In both Naruto and DxD, the characters are constantly striving to become stronger. Fanfictions often combine these two elements, showing how characters from one series adapt to the power - up systems of the other. For example, Naruto characters might learn about the Sacred Gears in DxD.
One common theme is the clash of different worlds. Spiderman's world of superheroics and the supernatural world of High School Dxd collide. Another could be friendship, as Spiderman might form new bonds with the characters in the High School Dxd universe.
One idea could be an OC who discovers their God Slayer magic in a dangerous situation at the high school. For example, a sudden attack by a stray devil forces the OC to tap into this unknown power within them to protect their friends. This could lead to a whole new arc in the fanfiction where the OC has to learn to control and master the God Slayer magic while also dealing with the regular high school DxD drama like relationships and rivalries.
A prevalent theme is the adaptation to a new environment. Dean, coming into the High School DxD universe, has to adapt to the different rules, cultures, and power dynamics. There are also themes related to loyalty and betrayal. In many fanfictions, Dean has to figure out who he can trust among the various characters, some of whom might be plotting against him while others are true friends.