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.
One idea could be that Naruto characters attend the Highschool DxD school. Naruto could use his shadow clone jutsu to cause all kinds of chaos during classes or P.E. Another is that some of the Highschool DxD devils could be interested in Naruto's chakra and try to make a deal with him. Also, the ninjas from Naruto could introduce their unique fighting styles to the students at Highschool DxD, leading to some epic battles in the schoolyard.
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.
One idea could be that Harry Potter somehow gets transported to the world of Highschool DxD. He could use his magic skills in the unique battles and challenges there. For example, he might use spells like Expelliarmus to disarm the devils or fallen angels during a fight.
One idea could be that Harry Potter, while exploring some new magic, accidentally opens a portal to the DxD world. He then has to adapt to the new rules and powers there. Maybe he teams up with Issei and they use a combination of Hogwarts magic and Sacred Gears to fight off a common enemy.
One plot idea could be the characters from Harry Potter learning about the different types of power and ranking systems in DxD. For example, Harry might be intrigued by the concept of Sacred Gears and try to figure out if there's anything similar in the wizarding world.
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.
Perhaps Natsu's presence could disrupt the balance between the three factions in the Highschool DxD world. His strong sense of justice might lead him to oppose some of the more underhanded dealings among the devils, angels, and fallen angels. This could start a chain of events where he has to prove himself and his beliefs to the various characters.
One idea could be that Inuyasha gets transported to the world of Highschool DxD. He might mistake the devils and angels there for demons from his own world and start attacking them. This would create chaos and also lead to some interesting alliances as the characters from Highschool DxD try to figure out who he is and why he's there. Another plot could involve a shared enemy. Maybe a powerful demon from Inuyasha's world somehow makes its way to the Highschool DxD universe and starts causing trouble for both groups. They would then have to combine their powers and knowledge to defeat it.
One interesting plot could be the characters from Highschool DxD somehow ending up in the Bleach universe. Maybe Issei and his friends are mistaken for Hollows due to their demonic energy and are hunted by Soul Reapers. They then have to prove their innocence while also learning about the complex spiritual world of Bleach.
Well, there could be a plot where the Sacred Gears in Highschool DxD are somehow connected to the ninja powers in Itachi's world. Itachi discovers that some of the students with Sacred Gears have a latent ninja ability within them. He then decides to train these students to help them control and enhance their powers. In return, the students of Highschool DxD help Itachi understand the complex social and power dynamics of their world, which is very different from his own.