One possible plot could be that Harry Potter, while on a new magical adventure, somehow gets transported to the world of Highschool DxD. He could use his unique magic skills to help the characters in Highschool DxD deal with their own supernatural threats. For example, his Patronus charm might be effective against some of the evil creatures in the DxD world.
Well, perhaps in this crossover fanfiction, the Hogwarts houses could be combined with the peerage system in Highschool DxD. Harry, being a natural leader, could form his own peerage with some of the students from Hogwarts and some characters from Highschool DxD. They could then compete in a magical tournament that combines elements of both worlds, like a Quidditch - like battle but with the devils' and angels' powers also in play.
One possibility could be the magic systems interacting. In Harry Potter, magic is more about spells and potions, while in Highschool DxD, it's more about the power of Devils and Angels. If they were in a crossover, perhaps the characters from Highschool DxD could learn the structured magic of Harry Potter, like Hermione teaching Issei how to brew a Polyjuice Potion. This would create some really fun and chaotic situations.
One possible interesting plot could be Sasuke arriving in the Highschool DxD world during a time of chaos. Maybe there's a new threat that neither the devils nor the other factions can handle easily. Sasuke's ninja skills and Sharingan would give him an edge in combat. He could form an alliance with Issei and his group, teaching them some ninja techniques in return for learning about the supernatural powers in their world.
Well, a cool plot might involve Issei from Highschool DxD being sent to the Naruto world. He could learn ninjutsu there and then bring some of those skills back to his own world. He might befriend Naruto and they could work together to fight a common enemy that has somehow crossed between the two worlds, like an evil group that wants to take over both the ninja world and the world of devils and angels.
One possible plot could be that Ichigo somehow gets transported to the world of High School DxD. He might initially be seen as a threat by the devils and angels there due to his unknown powers. But as they learn about his motives of protecting people, they start to work with him. Ichigo could also teach the characters in High School DxD about his unique fighting style and the concept of Hollow and Soul Reaper powers, leading to new and exciting battles against common enemies.
One interesting plot could be Naruto being summoned to the world of Highschool DxD. His unique ninja skills and chakra would be a huge shock to the devils, angels, and fallen angels in that world. He could end up forming an unlikely alliance with Issei, perhaps teaching him some of his ninja techniques in exchange for learning about the Sacred Gears. They could then face a common enemy together, like a powerful devil who wants to use Naruto's power for his own evil plans.
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.
The plot could involve Ben's aliens having an unexpected effect on the magic in the Highschool DxD world. His alien DNA might interact with the devil or angel powers in strange ways. For instance, if he transformed into Heatblast near a magic spell, it could cause the spell to go haywire. And his presence could also disrupt the power balance between the different factions in the DxD world, leading to new alliances and rivalries.
Well, it could be about Ranma getting transferred to the Highschool DxD world. Maybe he'd get involved in the supernatural battles there, using his martial arts skills. He might team up with Issei and the others, and his unique curse could add an interesting twist to the story. For example, his transformation between male and female forms could lead to some comical or strategic situations during fights.
A plot could involve a threat that endangers both universes. So, Harry Potter and his friends team up with Issei and his group. They start by trying to understand each other's worlds and powers. As they do, they find that their combined magic and skills are the key to defeating the common foe. This could involve things like the Hogwarts students teaching the Highschool DxD characters about complex spells, while the Highschool DxD characters share their combat techniques and unique powers.
The magic system in Harry Potter could be integrated by having characters from Highschool DxD learn spells. For example, Issei could learn the Wingardium Leviosa spell to lift objects during battle, giving him an unexpected advantage. The Portkey concept could also be used to transport characters between the two worlds quickly.