Well, it could be a very interesting and unique fanfiction. Naruto's world of ninjas and his values like loyalty and perseverance could blend with Ravel's character traits. Maybe they meet in some sort of crossover event. Naruto might be attracted to Ravel's charm, and they could start a relationship that overcomes the differences in their origins.
One common theme is probably the development of their relationship. It might show how they grow closer over time, from initial acquaintance to falling in love and finally getting married.
You can try searching on popular fanfiction websites like Archive of Our Own (AO3) or FanFiction.net. Just type in the relevant keywords 'Issei Marries Ravel' in their search bars, and you might find some interesting stories.
One common theme is the exploration of different powers. Naruto has his chakra - based ninjutsu while Ravel has her own set of abilities. Fanfictions often explore how these powers interact and complement each other. For example, Naruto's shadow clone jutsu could be combined with Ravel's magic in creative ways.
There are likely some out there. Some fans might write about Naruto and Ravel in a more platonic or adventure - based way. They could explore how the two characters from different universes might interact in a world where they meet, perhaps teaming up to face a common enemy or solve a mystery without any inappropriate undertones.
One possible plotline could be the challenges they face due to coming from different worlds. Naruto's ninja world and Ravel's world (whatever it may be) might have different cultures, values, and power systems. They would have to overcome these differences to build a strong marriage.
It could be Sasuke's sister, if there was one in the fanfiction world. She might share some of Sasuke's traits but also have her own unique personality that attracts Naruto.
No. In the official Naruto and High School DxD universes, there's no relation between them. But in fanfic, the writer can make up any relationship they want. It might be for the sake of adding an interesting twist to the story, like bringing together the ninja world of Naruto and the devil - based world of High School DxD in a new family - centered way.
Rather than seeking fanfictions with inappropriate elements, you could try to find Naruto fanfictions that revolve around his training, his battles against powerful foes, or his attempts to bring peace to the ninja world.
I'm sorry I can't directly recommend specific 'Naruto and Ravel marriage fanfiction' as there are so many out there. You can try searching on fanfiction websites like FanFiction.net or Archive of Our Own. Look for ones with high ratings and positive reviews from other readers.