One plot idea could be that your OC self - insert discovers a hidden power within themselves that is connected to an ancient prophecy in the High School DxD universe. This power makes them a target for various factions, both good and bad. Another idea is that they accidentally stumble upon a secret alliance between different species that is planning something big, and they have to decide whether to expose it or get involved.
Your OC could be a transfer student who has a strange connection to one of the main characters from their past that they don't remember at first. As they start to recall their memories, they uncover a dark secret that could put the entire school in danger. Also, they could get involved in a tournament where the prize is something extremely valuable, like a powerful artifact or a chance to make a wish. But the tournament is full of traps and opponents with their own motives.
Well, you can start by looking at the existing characters in High School DxD for inspiration. Your OC could be a mix of different aspects of those characters. Regarding their appearance, it should be something that stands out but also fits the overall aesthetic of the show. For their role in the fanfic, they could be a new student who shakes up the power dynamics at the school. Or perhaps a mysterious figure who has their own agenda and gradually reveals it as the story progresses.
A possible plot is that the OC is a misfit who doesn't fit into the typical categories in the High School DxD world. They might be a human with no obvious supernatural powers at first, but through their wits and determination, they manage to form alliances with different factions. They could use their knowledge of human technology and psychology to gain an edge, and along the way, they develop a unique power or ability that no one expected.
A good plot for Highschool DxD self - insert fanfiction might be that the self - insert character is actually a reincarnation of an ancient and powerful being. They start off not knowing this, but as the story progresses, they gradually unlock their past memories and powers. This could lead to them having to make choices between their new life in the Highschool DxD world and their old responsibilities from their past life.
One idea could be an OC who is a gamer in the High School DxD world and uses their gaming knowledge to strategize in battles. For example, they might use their understanding of RPG leveling systems to level up their own powers faster than others.
One plot idea could be that Naruto, with his ninja skills, gets transferred to the High School DxD world. He might accidentally join the Occult Research Club and use his Rasengan to defend the club from various threats. His positive attitude and never - give - up spirit could inspire the DxD characters and also attract the girls in the harem, leading to some comical and action - packed situations.
A great plot idea is that the self - insert vampire is actually a lost member of a forgotten clan in the Naruto world that has vampire - like traits. They are initially feared by the other characters but gradually prove themselves to be an ally. Or perhaps the vampire is from another dimension entirely and is accidentally pulled into the Naruto universe. They then use their unique abilities to help solve a major crisis in the world, like a war or an impending doom caused by a powerful enemy.
A great plot for a Highschol DxD OC fanfic could involve time travel. Imagine your OC accidentally travels back in time and has to interact with the ancestors of the current characters. They might change something in the past that has a huge impact on the present DxD world. Also, a plot where the OC is cursed by an unknown enemy. To break the curse, they have to complete a series of dangerous tasks across different realms in the DxD universe. And what about a plot where the OC starts a club in the school that challenges the existing power structures? It would be full of drama and conflict.
You could write a fanfic where the two worlds collide due to a powerful magic or jutsu gone wrong. The characters from both Naruto and High School DxD have to team up to fix the problem. This would bring together different personalities and abilities. For instance, Issei's perverted nature could be a source of comedic relief while Naruto's determination could inspire the DxD characters to fight harder.
One idea could be a crossover with 'Fate/stay night'. You could have the characters from High School DxD enter the Holy Grail War. Issei could use his boosted gear in new and exciting ways against the Servants. Another idea is a crossover with 'Naruto'. The ninjutsu from Naruto's world would be a cool addition to the battles in High School DxD. For example, Sasuke's Sharingan could create some interesting interactions with the devil eyes in DxD.
You could also base the fanfic on a tournament arc. All the characters from the series participate in a high - stakes tournament with unique rules. This would allow for a lot of action - packed scenes and strategic battles. The main character could face off against some of the toughest opponents and discover new things about their powers along the way.