A really interesting concept could be that Natsu somehow gets enrolled in the high school in High School DxD. He then has to navigate the high school life while also hiding his true identity as a dragon slayer. His hot - headed nature could get him into all sorts of trouble with the different factions in the school, but also earn him some unexpected allies.
One idea could be a crossover where Natsu finds himself in the world of High School DxD and has to deal with the devils and angels there. He might use his fire magic in unique ways to combat the various threats in that universe.
One plot idea could be Natsu accidentally ending up in the Highschool DxD world and having to adapt to the different rules and magic systems there. He could make friends with some of the main characters and they could team up to fight a common enemy that threatens both his world and the Highschool DxD world. Another idea is that Natsu's unique brand of magic could be a mystery to the characters in Highschool DxD and they try to figure out how it works while also teaching him about their own powers.
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.
Well, you could have a story where Mordred has a past connection to some of the powerful families in the Highschool DxD universe. Maybe she's on a quest to uncover something about her heritage that's linked to that world. And during her journey, she falls in love with one of the characters in an unexpected way. For example, she starts off as rivals with Rias but then they develop a deeper bond as they face challenges together.
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.
One idea could be to focus on the characters' relationships outside of the main plot. For example, how the students interact during normal school days, like study groups or after - school clubs. Another idea is to create an alternate universe where the rules of the dxd world are slightly different in the high school setting, which could lead to new adventures and challenges for the characters.
One interesting crossover could be a battle scenario where Natsu's fire - based powers from Fairy Tail are pitted against the devils in Highschool DxD. His dragon slayer magic might give him an edge against some of the lower - level devils.