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.
There could well be. Fanfiction writers often like to explore crossovers and unique concepts. However, not all fanfic writers might be interested in combining Naruto with 'Highschool DxD' in a certain way, so there are likely to be some that fit the 'Naruto not a devil highschool dxd fanfiction' idea. You might have to dig a bit through different fanfic archives to locate them.
In some Highschool DxD and Naruto crossover fanfictions, Naruto might be depicted as a god - like figure. This could be due to his extremely powerful abilities in the Naruto universe, such as his vast chakra reserves, multiple jutsu, and his status as a hero in his own world. Writers may take these aspects and amplify them in the Highschool DxD setting, where the power levels and the concept of beings with god - like powers already exist. So, it's possible for Naruto to be seen as a god in such fanfictions.
There's also 'Rise of the Unseen'. This fanfiction stands out because it focuses on some of the lesser - known characters from the Highschool DxD universe and gives them a chance to shine. It has a lot of action scenes that are described vividly, and the relationships between the characters are complex and interesting.
One great Irina fanfiction is 'Irina's New Path'. It explores her character development in new and interesting ways, showing her growing in power and relationships. Another good one is 'Irina in the Human World' which focuses on her experiences outside of the supernatural realm of Highschool DxD, giving a fresh perspective on her character.
In such a fanfiction, it could be that Issei is bashed because he represents a different kind of hero compared to what Naruto stands for. Naruto is all about self - sacrifice and believing in others no matter what, while Issei is more about his own desires and power - seeking in a way. So when these two worlds collide in the fanfic, Issei might be made to look bad in comparison. Maybe the story has Naruto characters like Sasuke or Kakashi giving Issei a hard time because they don't approve of his behavior.
A fanfiction idea could be that Issei and Xenovia get transported to a different dimension. In this new world, they are separated from their friends and have to find a way back. Issei tries to keep Xenovia's spirits up as they encounter various strange creatures. They discover new powers within themselves during their journey. Xenovia might also learn more about Issei's pervy side and they have some comical moments while still trying to figure out how to return home.
Yes, there are some interesting ones. You can check out fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'rikudou naruto highschool dxd' and you'll likely find a variety of stories with different takes on combining the elements of Naruto and High School DxD.