Some popular themes include the power dynamics within the harem. For example, some members might be more powerful than others, and how they use or don't use their power in the relationship is interesting. Additionally, the cultural and mythological elements from the 'High School DxD' universe are often explored in these fanfics, such as different demon clans and their traditions.
First, you need to have a good understanding of the 'High School DxD' universe. Know the characters, their powers, and the overall setting. Then, decide on the main characters for your harem. Next, think about the plot. It could start with an interesting event that brings the protagonist and the harem members together. And don't forget to add some unique twists to make it your own.
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.
There are also fanfics that might introduce new magical elements or abilities to the characters. Maybe a fanfic where Rias discovers a new power within herself and has to learn to control it, all while dealing with the regular chaos in the school and the underworld.
First, know the original 'High School DxD' world well. Understand the personalities of the characters, their relationships, and the overall lore. This will help you write in a way that feels consistent with the source material.
One possible storyline could be that Queen Lemon is a lost princess from a hidden land in the Naruto world and she ends up in the High School DxD universe. There, she has to find her way back home with the help of the characters from both series. For instance, Issei from High School DxD could use his unique powers to protect her while Naruto teaches her some ninja skills for self - defense.
Typically, you'll see the main characters from the original High School DxD series like Issei Hyoudou. He might be in a different role in the fanfic, perhaps more involved in the'slave' aspects of the story. Rias Gremory could also be a main character, maybe as a figure of power who has'slaves' under her command.
One interesting storyline could be about the characters' unique power development within the fanfic's universe. For example, a character might gain new powers related to their'slave' status that gives them an edge in battles or challenges. Another could be a story of friendship and loyalty that develops between the main characters despite the complex'slave' relationships depicted in the fanfic.
One 'high school dxd oc harem fanfic' that stands out is 'My OC's Harem in High School DxD'. It starts with the OC being a bit of an outsider but quickly gets integrated into the world. The harem elements are very engaging as the OC has to deal with different personalities among the female characters. There are also some really cool fight scenes and plot twists that make it an interesting read.
In a harem fanfic, the characters can interact through a cultural exchange of sorts. Naruto could introduce the concept of ninja clans and honor to the High School DxD characters. The female DxD characters might find this new concept fascinating and be attracted to Naruto because of it. Also, they could interact in a rescue mission. If one of the DxD characters or Naruto gets captured, the others come to the rescue. During this process, the female DxD characters would see Naruto's loyalty and courage, which would make them more interested in him as part of the harem. And of course, there could be parties or social events where they all mingle. Naruto's outgoing and friendly nature would make him popular among the female DxD characters, leading to more interactions and the development of the harem.