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.
There could be a fanfiction where Naruto's chakra is somehow connected to a powerful magic source in Ravel's world. This connection causes chaos at first but then they figure out a way to use it to their advantage. Maybe they use it to restore balance in both their worlds that were being threatened by a common evil. They travel between the two worlds, gathering allies and knowledge along the way.
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.
The Ravel Twins' magic might be able to influence Naruto's shadow clones. They could use their magic to give the clones additional abilities, like the power of flight or the ability to phase through objects. In return, Naruto's clones could be used as decoys or additional manpower in battles against magical foes. His chakra could also provide a stable source of energy for the twins' more draining magical spells.
Often, Naruto himself is a popular character to pair with characters from Ravel's world. His strong will and determination make him an interesting addition to any crossover story. Another popular pairing could be Sasuke from Naruto with a powerful character from Ravel. Their brooding natures and strong powers can create an interesting dynamic.