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.
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 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.
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.
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.
One idea could be a crossover where Naruto characters end up in the Highschool DxD world and have to deal with the devils and angels there. For example, Naruto could use his shadow clone jutsu in a battle against a group of devils, creating chaos. Another plot idea might be that a Leviathan character from Highschool DxD travels to the Naruto world and tries to learn ninjutsu. It could lead to some comical situations as they struggle with chakra control.
One idea could be that Naruto, through some magical or interdimensional event, ends up in the world of Highschool DxD. He could use his ninja skills and kind heart to win over one of the female characters, like Rias. Maybe he helps her with some underworld threats using his shadow clone jutsu, and in the process, a romantic bond forms between them.
One idea could be that Naruto somehow gets transported to the High School DxD world and has to use his ninja skills in the battles there. Maybe he teams up with Issei and they combine their powers in a unique way.
One plot idea could be Naruto being transferred to the Highschool DxD world and having to adjust to the new school environment while also dealing with the supernatural elements. He could form an unexpected friendship with Ophis, perhaps over their shared experiences of being different from the norm in their respective worlds.
One plot idea could be a character from Naruto ending up in the Highschool DxD world and having the Rinne Sharingan. They could use its powers in the battles against the devils and fallen angels in the new world.
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.