A great plot idea could be a tournament - style story. The characters from Fairy Tail and Highschool DxD are forced to participate in a tournament. It could be organized by some powerful being that wants to see which universe has the stronger fighters. This would lead to some epic battles like Gajeel vs Kiba, with their different fighting styles of iron dragon slayer magic and swordsmanship. And during the tournament, there could be some interesting interactions between characters from both universes, like Akeno being curious about Wendy's healing magic.
Well, perhaps the Fairy Tail guild could end up in the Highschool DxD universe. The Fairy Tail members would then have to adapt to the different rules and powers in that world. Like, Gray's ice magic might be a unique asset against some of the hotter - tempered devils. And Lucy could use her celestial spirits to gain an edge in battles. Also, the concept of Fairy Tail's guild spirit could influence the way the devil clans in Highschool DxD operate.
One idea could be that the magic from Fairy Tail somehow leaks into the world of Highschool DxD. For example, Natsu's fire magic could be mistaken for a new type of devil power. Another could be a crossover of characters where Erza teams up with Rias to fight a common enemy. Maybe a powerful dark guild in Fairy Tail teams up with some rogue devils in Highschool DxD.
Maybe the dragons from Fairy Tail are related to the dragons in Highschool DxD in some ancient way. So, characters like Igneel might have a connection to the dragon in DxD. This could lead to a search for the truth about their origins and a fight against a common enemy that threatens both worlds.
Maybe there could be a big competition at the highschool, like a magical sports event. Different characters from Fairy Tail would form teams and compete, with rivalries and friendships being tested. For example, Natsu's team against Gray's team, and there could be some comical moments as well as intense battles.
A possible storyline is about a character from Fairy Tail who transfers to the Highschool DxD school. They have to adapt to the new environment which is full of devils, angels, and fallen angels. Along the way, they make new friends and enemies, and their Fairy Tail magic causes quite a stir among the students.
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.
Perhaps the OC is a skilled musician and Sona, who has an appreciation for the arts, meets the OC at a school concert. They start collaborating on music projects and their relationship blossoms from there. The OC could also have some sort of hidden past related to the supernatural world that Sona helps them uncover.
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.