Well, there could be a 'naruto leviathan fanfiction' where the leviathan is actually a form of a powerful jutsu gone wrong. Naruto and his friends have to find a way to reverse the jutsu and stop the leviathan from wreaking havoc. It could involve a lot of teamwork and Naruto using his new - found skills to combat the beast. This type of story would be full of action and adventure as they race against time.
There might be a fanfiction where Naruto is summoned to the devil world by Serafall. In this story, Naruto could be seen as an exotic being, and he has to navigate the complex politics and power struggles in the devil world. Serafall might take Naruto under her wing, teaching him about the devil world while Naruto teaches her about his ninja way. It could be a story full of adventure and cultural exchange between two very different universes.
Perhaps in the fanfiction, the leviathan is a sentient being from a different dimension. Naruto forms a bond with it not just as a summon but as a friend. They communicate telepathically, and the leviathan helps Naruto understand some of the deeper secrets of chakra by showing him how it manipulates energy in its own dimension.
There might be a fanfiction where Naruto meets Katerea Leviathan in a different dimension. In this story, Naruto has to learn about the new world Katerea comes from and they end up having to work together to find a way back to Naruto's world. It could involve exploring strange new places and encountering odd creatures along the way.
One interesting fanfic could be where Naruto somehow enters the world where Serafall Leviathan is a queen. In this story, Naruto might have to adapt to the new world's magic system while also dealing with the political intrigue around the queen. Maybe he becomes an ally to her and they face a common enemy together.
One common theme is the clash of cultures. Naruto comes from a world of ninjas, and Serafall from a magical, hierarchical world. Another theme could be power dynamics. As a queen, Serafall has a certain level of authority, and Naruto has his own power as a ninja. Their interactions in terms of who has more influence can be an interesting plot point.
Well, one way is through character exchanges. Imagine a character from Naruto like Naruto himself ending up in the Highschool DxD universe and meeting Leviathan. He could introduce his ninja skills to that world and also learn about the devil powers. Another way is to have a common enemy. For instance, a new threat that can only be defeated by combining the unique powers from all three, like the Rasengan combined with a Highschool DxD power against an enemy threatening Leviathan.
The leviathan could be part of a prophecy that Naruto doesn't fully understand at first. When he summons it, he discovers that the leviathan's actions will either save or destroy the ninja world, depending on what Naruto does next. This creates a high - stakes situation and a great plot twist.
One interesting crossover could be a story where the power - up systems of Naruto and Highschool DxD are combined. For example, a character from Highschool DxD might learn how to use chakra like in Naruto. And Leviathan could be a powerful entity in this new world, perhaps with abilities influenced by both Naruto's jutsu and the unique powers in Highschool DxD. This could lead to really cool battle scenes and character development.