One idea could be that Naruto characters attend the Highschool DxD academy. For example, Naruto himself could be a new student with his unique ninja skills. He might form an unlikely friendship with Issei. Another idea is a battle tournament where ninja techniques are pitted against the devilish powers in the DxD world.
A cool plot idea could be a tournament where characters from both Highschool DxD and Naruto participate. The ninjas would show off their jutsu and the devils would display their powers. Also, imagine if Naruto got a Sacred Gear. He could use it to enhance his Rasengan or even create a new jutsu that combines his ninja abilities with the power of the Sacred Gear. Or perhaps, the female characters from Highschool DxD like Rias could teach some of the kunoichi in Naruto about different ways of using their femininity as a power, which could lead to some interesting character development for the kunoichi.
Maybe in the fanfiction, the Campione characters are invited to the Highschool DxD world to participate in a special tournament. They bring their own unique skills and knowledge, and during the tournament, they form unexpected alliances with the devils, angels, and fallen angels. This would allow for a lot of character interactions and development.
One idea could be to cross it with a magical girl anime. The main character from Highschool DxD could find himself in a world full of magical girls and form a harem with them. They could fight together against common enemies while also dealing with the typical harem situations like jealousy and misunderstandings.
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.
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.
One idea could be to have the characters from High School DxD cross over into a world ruled by different gods. They could be tasked with a quest to restore balance. For example, Issei could use his Boosted Gear in a realm where the gods' powers are waning and his power might be the key to saving the day.
Perhaps a story where Kuroka transfers to the high school in the Highschool DxD setting and she meets Naruto who is also there for some reason. They could form an unlikely alliance against common enemies. Kuroka's cat - like abilities and Naruto's combat prowess would make for an interesting team - up. And they could also have some comical moments as Naruto tries to understand the strange rules and hierarchies of the Highschool DxD world.