Maybe Spider - Man's identity is mistaken in the Highschool DxD world. The characters might think he is some sort of new supernatural being. This could lead to a series of comical misunderstandings as he tries to explain his powers and where he comes from while also dealing with the various power struggles and intrigues in the Highschool DxD universe.
One way Spiderman could fit in is if he was brought there by some sort of magical or scientific accident. He could then use his spider - sense to detect the hidden motives and powers of the characters in Highschool DxD. He might get involved in the power struggles between the devils, angels, and fallen angels, trying to maintain a balance. His suit could also be modified with some magic or technology from the Highschool DxD world to give him new abilities.
A Spiderman and Highschool DxD fanfiction might involve Spiderman teaming up with Issei. Spiderman could teach Issei some new combat moves based on his acrobatic fighting style. And Issei could introduce Spiderman to the unique aspects of the Highschool DxD world, like the different factions and the power of Sacred Gears. They could go on adventures together, facing off against powerful foes that threaten both their worlds.
One tip is to start with a conflict. It could be an internal conflict within your OC, like struggling with their newfound powers. Or it could be an external conflict, such as a new enemy threatening the peace between the devils, angels, and fallen angels. Another thing is to incorporate the existing Highschool DxD lore. This gives your fanfiction a sense of authenticity. For example, use the different rankings of devils and angels in your plot.
Well, a plot could involve Broly being part of a special program in the Highschool DxD setting. He might be there to learn about controlling his power in a more civilized way. As he tries to fit in, he could befriend some of the main characters from DxD. For example, he could team up with Issei and his friends to compete in some sort of school tournament that has magical and combat elements. This would lead to some epic battles and also some character development for Broly as he learns about friendship and cooperation.
Well, one interesting plot twist is when the Nine - Tails in Naruto is somehow linked to a powerful devil in Highschool DxD. Imagine the chaos and new power - sharing or battling that could occur. It completely flips the script on how we see both the Naruto and Highschool DxD elements. Also, there could be a plot twist where the power - leveling system in Highschool DxD is reversed for Naruto characters, making them struggle in a new way and forcing them to adapt in unexpected ways.
A plot could also focus on a new character who is a hybrid of the two universes. They have the powers of both a devil from High School DxD and a wizard from Harry Potter. They struggle to find their place in either world as they are too different from the norm in both, and they embark on a journey to create their own identity and purpose.
One possible interesting plot could be the characters from Naruto and Highschool DxD universes being forced to attend a special high school together. Naruto characters like Naruto himself might be intrigued by the unique magic and devil - related things in the Highschool DxD world. And the devils from Highschool DxD could learn a lot about chakra and ninja techniques from the Naruto crew. They could have joint battles against a common enemy that threatens both their worlds, and in the process, form unexpected friendships and rivalries.
One possible plot could be that Naruto, with his unique ninja skills, is transferred to the Highschool DxD world. He could form an unlikely alliance with the main characters there. Maybe he uses his shadow clone jutsu to help out during some of the school's chaotic events or battles against the devils and fallen angels, adding a new and unexpected element to the Highschool DxD's power dynamics.
One idea could be exploring new power-ups for the characters. For example, Issei could discover a hidden ability within his Boosted Gear that allows him to time - travel briefly during battles, adding a new strategic element. Another could be focusing on the relationships in the future. Maybe Rias and Issei have a family and the story could revolve around their children's upbringing in the supernatural world.
One idea could be a new character joining the school who has a unique supernatural ability that no one has seen before. This could disrupt the existing power dynamics and create all sorts of interesting interactions and challenges.