Another storyline could be a 'what if' scenario. What if Issei was born as a devil instead of a human? This would completely change his character arc and the way he interacts with others. He might have different goals and motives, and it would be fascinating to see how this new version of Issei would fit into the complex world of High School DxD.
Issei is definitely one of the most popular. He's the main character in the original, so fans often like to continue his story or put him in different situations in fanfiction. His growth from a perverted human to a powerful devil is a great base for many fan - created stories.
A fanfiction might explore a story where the Occult Research Club members are all transported to a different dimension. In this new world, they have to find a way back home while facing new enemies and challenges. Rias could use her leadership skills in a different way, and Kiba might discover new powers within himself to protect the group. This would also test the bonds between the characters in a whole new setting.
Another could be Mittelt forming an unlikely friendship with a character from a different faction in the Highschool DxD universe. For example, she befriends a member of the fallen angels' group that her side is usually at odds with. This could lead to all sorts of plot twists as they try to bridge the gap between their two groups. And it would be really cool to see how Mittelt grows and changes through this relationship.
Well, a great storyline could involve a slave character who has a unique relationship with one of the main characters from Highschool Dxd. Maybe they start off as just a slave but through shared experiences, like fighting together against a common enemy, they develop a deep bond that changes the whole dynamic of the story and challenges the traditional master - slave relationship in that world.
One interesting storyline could be about a new character who joins the school and has a unique power that changes the power dynamics among the existing groups. This new character might start as an outcast but gradually earns the respect of the others through their actions.
Another possible storyline is a love triangle between a Saitama Highschool student, a character from DxD, and another student from their own school. This would add an element of drama and emotion to the fanfiction, as they navigate their feelings while also dealing with the supernatural elements of the DxD world.
One interesting storyline could be about a new character who has a unique power that shakes up the power dynamics in the Highschool DxD world. For example, a character with the ability to nullify all magic for a short time, creating new challenges and alliances.
One interesting storyline could be about a new character in the Futa Highschool DxD universe who has unique powers and has to navigate the complex social and power dynamics. For example, they might be a half - demon, half - angel and face discrimination from both sides.
A lot of fanfictions explore Irena's relationships with the other main characters. Maybe she forms an unlikely friendship with a character who is very different from her. This can lead to some really fun and heartwarming interactions as they learn from each other and grow together.
One interesting storyline could be Harry Phenex's adventures in the Highschool DxD world while trying to hide his true identity as a Phenex. He might have to deal with the various factions in the school and their power struggles.