Another plot could be that Ravel discovers a way to summon Naruto to her side for a special mission. She's heard of his great strength and thinks he can help. As they work together, Naruto starts to learn about the different powers and hierarchies in the DxD universe. And they gradually develop a friendship that might turn into something more as they face challenges together. Naruto's chakra could also interact in unexpected ways with the magic in the DxD world.
Perhaps there's a prophecy in the Highschool DxD world that mentions a 'foreign hero' which turns out to be Naruto. Ravel is tasked with finding and guiding him. Naruto, being his usual self, is eager to help. They then embark on a journey to stop a great evil that threatens both the DxD world and, in some way, Naruto's own world. Along the way, they exchange knowledge, Naruto teaching Ravel about ninjutsu and Ravel teaching Naruto about devil magic.
One idea could be that Naruto, through some sort of interdimensional rift, ends up in the world of Highschool DxD. Ravel could be intrigued by this strange new being with his unique powers. Maybe Naruto has to team up with Ravel to face a common enemy in the DxD world, like a rogue group of devils. His ninja skills would be a novelty there and could help them in battles.
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.
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.
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.
It could be a powerful devil who doesn't like the idea of an outsider like Naruto in their world. This devil might see Naruto as a threat to the power balance and wants to eliminate him before he can form strong alliances. He could be a high - ranking noble devil who has his own dark plans for the DxD world and Naruto's presence is ruining it.
One idea could be that Ichigo is somehow transported to the High School DxD world and has to deal with the Devils and Angels there. He might use his Soul Reaper powers in unique ways to solve the problems in this new world.
Perhaps there could be a crossover of the two schools. The students from Beacon Academy and Kuoh Academy could have an exchange program. Yang might get into some friendly (or not so friendly) competitions with the devils in terms of strength, while Weiss could be fascinated by the more formal and complex social structure in the Highschool DxD world.
Maybe in the fanfiction, there's a magical event that links the world of Illya with the Highschool DxD universe. Illya's innocent nature could be a contrast to the more risqué and wild nature of some of the Highschool DxD characters. Her friendship with Issei could start off rocky but then develop into a strong bond as they train together to improve their respective powers.
One idea could be a character from Prototype having to adapt to the supernatural world of Highschool DxD. For example, Alex Mercer could end up at Kuoh Academy and have to deal with the devils and angels while also trying to figure out his own powers in this new context.
One idea could be an OC who is a transfer student with a unique power that no one in the Highschool DxD universe has seen before. This power could attract the attention of the main characters and lead to various adventures and alliances.