A plot could revolve around a cultural exchange. The Naruto characters visit the Highschool DxD world and vice versa. They learn about each other's cultures, like the ninja way in Naruto's world and the devil - angel - fallen angel society in Highschool DxD. This could lead to some comical misunderstandings at first, but then develop into strong friendships and alliances.
One idea could be a sports competition between the Naruto characters and the Highschool DxD characters. They could compete in something like a ninja - themed obstacle course that combines elements from both universes.
One idea could be Ravel's adventures in the school's magic club. She could be working on new spells and facing challenges like accidental magic mishaps. For example, she might try to create a spell to make plants grow faster for a school project, but instead makes them grow out of control and has to figure out how to reverse it.
Well, a plot could revolve around a mystery that involves both worlds. Maybe there is an ancient artifact that has the power to disrupt the balance between the Naruto universe and the Highschool DxD universe. Characters from both sides have to work together to find and protect this artifact. This would lead to a lot of character development as they learn about each other's cultures and values while on this quest.
One idea could be that Naruto somehow gets transported to the High School DxD world and has to deal with the devils and angels there while still having his ninja skills. He might use his shadow clone jutsu in unique ways during battles in the new world.
One idea could be a crossover where Naruto characters end up in the Highschool DxD world and have to adapt to the different rules regarding magic and devils. For example, Naruto could use his chakra in unique ways against the devils' powers.
Well, a cool plot could involve a character from Naruto getting transferred to the Highschool DxD universe. They could bring some ninja techniques and wisdom which would be really interesting in the context of the more modern and somewhat chaotic world of Highschool DxD. Or perhaps there's a mystery that only the skills of Naruto characters can solve in the Highschool DxD setting, like a strange demonic phenomenon that has ninja - like stealth and secrecy.
A plot could be that the Akatsuki from Naruto are after a powerful artifact in the Highschool DxD world that is related to Lucifer. This would bring the two universes into conflict as the devils and other supernatural beings in Highschool DxD would not want the Akatsuki to get their hands on it. Naruto characters like Sasuke or Kakashi could be pitted against the devils like Issei or Rias. And Lucifer could be in the middle, either trying to stop the Akatsuki for his own reasons or using them to gain more power in his own world.
Another idea is that a powerful artifact has been stolen and its pieces are scattered across the three worlds. Naruto, Gabriel, and the Highschool DxD characters must team up to find all the pieces. As they search, they encounter different challenges in each world. In Naruto's world, they face ninja clans who want the artifact for their own power. In Gabriel's world, they must deal with angelic politics. And in the Highschool DxD world, they have to fight off demons who also want the artifact.
One plot idea could be that Naruto is somehow transported to the world of Highschool DxD and meets Ophis. Maybe he has to help the characters in Highschool DxD deal with a threat using his ninja skills, and in the process, he forms an alliance with Ophis.
Well, a plot could involve Naruto having a delinquent past in his own world which somehow affects how he behaves in the Highschool DxD universe. He might be constantly getting into fights with the various factions because he doesn't fully understand their rules. And his rasengan could be a super - powerful move that both amazes and scares the characters in the new world.
One idea could be a tournament - style plot. Characters from all three universes are invited to a grand tournament. Naruto's ninjas, the devils from Highschool DxD, and Gabriel's angelic forces compete against each other in various challenges. This would showcase the different fighting styles and powers of each group.