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.
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.
One interesting story could be about a fairy leviathan that forms an unlikely friendship with a young mermaid. They go on adventures together, exploring the deep sea and encountering other magical creatures. The fairy leviathan uses its magical powers to protect the mermaid from various dangers.
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 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.
One idea could be a crossover where Naruto characters end up in the Highschool DxD world and have to deal with the devils and angels there. For example, Naruto could use his shadow clone jutsu in a battle against a group of devils, creating chaos. Another plot idea might be that a Leviathan character from Highschool DxD travels to the Naruto world and tries to learn ninjutsu. It could lead to some comical situations as they struggle with chakra control.
One interesting plot point could be the new challenges Percy faces against the Leviathan. Maybe he has to discover a unique weakness of the Leviathan to defeat it. Another could be the relationships that develop or change during this encounter, like his friendship with his comrades getting stronger as they face this huge threat together.
Some 'oc leviathan dxd fanfiction' stories might center around the OC having special powers or abilities that attract Leviathan's attention. Maybe the OC has a power related to water, similar to Leviathan's own abilities, which leads to an interesting bond between them. They could then team up to face common enemies in the DxD realm.
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.
One interesting concept could be Harry Potter's Animagus form interacting with the Leviathan. Maybe his Animagus form is a small creature like a mouse, and it has to find a way to communicate or avoid the Leviathan. Another idea is that the Leviathan has some sort of magical connection to Harry's Animagus ability, like it can sense when he is in that form. Also, perhaps Harry uses his Animagus form to explore a hidden world where the Leviathan resides, and discovers secrets about the wizarding world that were previously unknown.
One idea could be Leviathan using her unique powers in a new way to solve a mystery within the school. For example, she might use her water - based abilities to uncover hidden passages or secrets. Another could be a story where she forms an unexpected alliance with a character from a different faction to face a common threat.