Well, one interesting idea could be the characters experiencing different relationships. For example, when two characters swap bodies, they might have to deal with the other's friends and enemies in a whole new way. It can lead to some really funny or touching moments as they try to navigate the other's social life.
One idea could be to focus on the characters' relationships outside of the main plot. For example, how the students interact during normal school days, like study groups or after - school clubs. Another idea is to create an alternate universe where the rules of the dxd world are slightly different in the high school setting, which could lead to new adventures and challenges for the characters.
Rias is also a common character in these types of fanfictions. Her status and powers make for some really engaging body swap scenarios. For example, if she swaps with a less powerful character, it can show a different side of her as she has to adapt to the limitations of the new body.
Well, you could write a fanfiction where Fafnir discovers a hidden power within himself. He might struggle to control it at first but then learns to use it for good. Or perhaps a story that focuses on Fafnir's relationships with the other characters. For example, his friendship with Issei could be developed further, showing how they support each other in different situations. Also, a fanfiction about Fafnir getting involved in a magical tournament would be exciting.
A good concept for 'highschool dxd harem fanfiction' could be a time - travel story. The harem and Issei could be sent back in time and have to navigate the past while also dealing with their feelings for each other. They might meet younger versions of themselves or their predecessors. Another idea is to have a mystery element, where something strange is happening in the school and the harem has to solve it together. This could bring out different sides of each character and also develop their relationships further.
One idea could be exploring a new character's journey in the Highschool DxD human world. Maybe a transfer student who has a unique power or ability that shakes up the status quo among the devils, angels, and fallen angels. They could start off as an unknown and gradually form alliances and rivalries.
One idea could be a story where the characters from High School DxD are reimagined as toddlers, and their innocent interactions lead to comical and heartwarming situations. For example, Issei might be chasing after a butterfly instead of chasing girls in his usual way.
A third idea could be a fanfiction where Gaara becomes a devil himself. He could bring his ninja skills and his sand abilities into the devil society. This would change the power dynamics in the DxD highschool as he would be a very different type of devil compared to the existing ones, with his own moral code and way of doing things.
One idea could be to create an alternate universe where the main character has a different set of powers. For example, instead of the traditional Sacred Gear, they could have a power related to time manipulation. This would change the dynamics of battles and interactions with other characters.
A third idea could be a time - travel fanfiction. For example, a character from Highschool DxD travels back in time to the era of Bleach and meets Ichigo when he was still developing his powers. This could lead to some interesting interactions and changes in the timeline of both universes.
You could write about an OC who is a half - angel, half - devil. Their internal struggle between the two natures and how they fit into the world of Highschool Dxd, trying to find their place among the devils, angels and fallen angels. It could explore themes of identity and acceptance.