There could be. Fanfiction writers are known for their wild and creative crossovers. Given the popularity of Highschool DxD and the unique concept of Guyver, it's entirely possible that an Issei - Guyver fanfiction exists. You might want to check popular fanfiction websites like Archive of Our Own or FanFiction.net. Just search for relevant keywords like 'Highschool DxD Issei Guyver' and see if any relevant stories pop up.
The OC, as the Son of Sparda, could fit in by being a new and powerful entity. He might be initially seen as an outsider, but his powers would quickly draw attention. He could become involved in the power struggles between the different factions in Highschool DxD. For example, the devils might try to recruit him, while the angels might be wary of his presence. His unique abilities would also make him a valuable ally or a fearsome opponent. And in the high school setting, he could be a bit of an enigma to the other students, both human and supernatural.
The female characters from Highschool DxD, like Rias, would be intrigued by the unique personalities of the Naruto characters. Rias might see potential in some of them to join her peerage. And the Naruto girls, like Sakura, would be a bit shocked at first by the more revealing outfits and flirty nature of some of the Highschool DxD characters. But over time, they would bond over things like protecting their friends and the school. The Naruto boys would respect the power of the Highschool DxD devils and might even try to incorporate some of their fighting strategies into their own ninja combat styles.
A third idea is that there's a mix - up with the curses in both worlds. Ranma's curse of changing into a girl when splashed with cold water gets combined with some of the magic in Highschool DxD. This creates a whole new set of problems and adventures for him as he tries to figure out how to deal with this strange new situation while also trying to keep his identity a secret from the new characters he meets.
One possible fanfiction could involve a crossover where the characters from Highschool DxD and Naruto meet in a unique world. Yasaka, being a powerful figure in the Highschool DxD universe, might interact with Naruto characters. For example, she could form an alliance with Naruto to fight against a common enemy that somehow threatens both of their worlds. Maybe a group of rogue ninjas from Naruto's world team up with some rogue devils from Highschool DxD.
One great Highschool DxD mercenary fanfiction is 'Mercenary of the Underworld'. It has an interesting plot where the main character, a mercenary, gets involved in the supernatural world of Highschool DxD. The action scenes are well - written and the character development is engaging.
One possible fanfiction could be about a crossover where the characters from Highschool DxD and Naruto are transported to a world where they meet Gabriel. Maybe they team up to fight a common enemy. For example, the devils from Highschool DxD and the ninjas from Naruto could combine their powers under Gabriel's guidance to defeat an ancient evil that threatens all three of their realms.
There is also a fanfiction called 'Ophis and the School Festival'. In this story, Ophis gets involved in the school festival preparations. It has a lot of humorous moments and also some great character interactions. For example, her interactions with Issei and the other members of the Occult Research Club are really well - written.
One great Highschool DxD Youkai fanfiction is 'The Youkai Princess in Kuoh Academy'. It has an interesting plot where a youkai princess enrolls in the academy and forms unique relationships with the main characters. The story blends the elements of the Highschool DxD world with youkai lore really well.
One great fanfiction could be where Murayama discovers Issei's true nature as a devil early on. This could lead to some really interesting adventures as she tries to come to terms with it. Maybe she starts to train with him to be able to protect herself in the dangerous world of devils and fallen angels.