A cool plot could be that the Highschool DxD characters are transported to the One Piece world. Issei, being the perverted yet brave character he is, might be really excited to see all the different female characters in One Piece. But he also realizes that they are in a dangerous world. Meanwhile, the One Piece characters are baffled by the strange powers and personalities of the Highschool DxD crew. They all have to work together to face the various pirates and threats in the One Piece world.
Luffy and Issei would be an interesting pair. Luffy's carefree and fearless nature combined with Issei's determination and his unique Sacred Gear abilities could lead to some really fun and action - packed scenes. They could learn from each other, like Luffy teaching Issei about his unwavering belief in his dreams and Issei showing Luffy some of the more strategic ways to use powers in the Highschool DxD world.
Well, one interesting concept could be the combination of the unique powers from One Piece and the supernatural elements of Highschool DxD. For example, a character with a Devil Fruit ability attending the high school in the Highschool DxD universe and having to deal with both the challenges of that school like the Rating Games and also the threats from the One Piece world such as the World Government or pirates.
One idea could be that the characters from Highschool DxD end up in the Hellsing universe. Maybe Issei and his friends are mistaken for new types of vampires due to their devil powers. They then have to team up with the Hellsing organization to clear their names and also fight against a common enemy that is threatening both the devil and human worlds in this combined universe.
A third idea is that there's a mix - up with the curses in both worlds. Ranma's curse of changing into a girl when splashed with cold water gets combined with some of the magic in Highschool DxD. This creates a whole new set of problems and adventures for him as he tries to figure out how to deal with this strange new situation while also trying to keep his identity a secret from the new characters he meets.
One way is to have the Devil Fruits play a major role. Imagine a character in the Highschool DxD world who has eaten a Devil Fruit and has to hide it from others while also using its powers to gain an edge in the Rating Games or in battles against other supernatural beings. For example, a Logia - type Devil Fruit user could transform into an element to avoid attacks during a fight in the schoolyard.
One possible fanfiction could be about Naruto somehow ending up in the world of Highschool DxD and meeting Moka. Maybe Naruto's skills and his never - give - up attitude would impress Moka and the other characters. He could teach them some of his ninja techniques, like the Shadow Clone Jutsu, which could be useful in battles against the devils and other supernatural beings in the Highschool DxD world.
One possible scenario in such a fanfiction could be Shirou Emiya's unique abilities from the Fate series being introduced into the Highschool DxD universe. His projection magic might cause quite a stir among the devils, angels, and fallen angels in that world.
The theme of friendship is quite common. For example, in a crossover with 'My Hero Academia', the characters from both series form new friendships and alliances. They learn from each other's strengths and weaknesses. Also, the theme of adaptation is present. Characters from Highschool DxD have to adapt to the new rules and environments of the crossover world. Say, if they cross over with a post - apocalyptic world, they need to figure out how to survive and use their powers in that new context.
One possibility could be that Ichigo's powers would make him a very unique and powerful character in the Highschool DxD universe. His zanpakuto abilities and high - level combat skills could give him an edge in the battles against the devils and other supernatural beings in that world.