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.
You can try fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'Highschool DxD Issei x Gabriel' on their search bars, and you should be able to find relevant fanfictions.
The fanfictions also allow for a deeper exploration of the characters' personalities. Issei's determination and Gabriel's grace can be further developed. Writers might give them new backstories or put them in situations that are not in the original anime. This gives readers a fresh perspective on the characters and their relationship, which is what makes these fanfictions so interesting.
One idea could be a tournament - style plot. Characters from all three universes are invited to a grand tournament. Naruto's ninjas, the devils from Highschool DxD, and Gabriel's angelic forces compete against each other in various challenges. This would showcase the different fighting styles and powers of each group.
One popular plot idea could be the OC being a new student at the school and catching Gabriel's attention. Maybe the OC has a unique power or ability that Gabriel is intrigued by. Another idea is a story where the OC is in some sort of danger and Gabriel has to protect them. This could lead to a deeper connection between them as they face challenges together.
To write a good OC, start with giving them a distinct personality. For example, if you want them to be a contrast to Gabriel, make them more impulsive or hot - headed. Also, give them a backstory. Maybe they come from a family with a dark secret in the supernatural world. This will add depth to their character.
In a fanfiction, it could go either way. Naruto has shown the ability to grow and adapt constantly. He has defeated some extremely powerful foes in his own universe. Gabriel, on the other hand, has powers that are more on the spiritual and divine side. If the fight takes place in the DxD world where there are rules and powers specific to that universe, Gabriel might have the upper hand. But if Naruto can find a way to use his chakra in a unique way against Gabriel's powers, he could also be the stronger one.
Perhaps Naruto is mistaken for a powerful devil or an unknown being in the DxD universe. Gabriel, being curious, approaches him. They start off on a rocky relationship as Naruto is confused by this new world. But as they get to know each other, they find out that their powers can be combined in unique ways. For example, Naruto's shadow clone jutsu combined with Gabriel's holy powers could create some really cool and unexpected attacks against their foes.
One idea could be that Naruto is somehow transported to the world of Highschool DxD. He could use his ninja skills and chakra in the new world, maybe even teach some of the characters there a thing or two about combat. For example, he could show Issei how to better control his power using chakra control techniques.
A possible crossover could involve the plotlines of both worlds. The threat of Akatsuki in Naruto could be related to some of the power struggles in Highschool DxD. For instance, if the Akatsuki were after the Sacred Gears for some reason. This would force the characters from both universes to team up, leading to some exciting battles and character development as they learn from each other's strengths and weaknesses.