One idea could be that Naruto characters are somehow transported to the world of Owari no Seraph. For example, Naruto himself could use his chakra to fight against the vampires in Owari no Seraph. His shadow clones would be a great asset in battles. Another plot could involve the sharing of powers between the two universes. Maybe some of the Owari no Seraph characters gain the ability to use ninjutsu, creating a whole new dynamic in their fight against the demons.
A crossover with 'Fullmetal Alchemist' would also be cool. Maybe the alchemical powers in that world could interact with the vampire and demon powers in 'owari no seraph'. The characters could be on a quest to find a way to either cure the vampires or use alchemy to stop the apocalypse that's happening in 'owari no seraph'.
A possible plot is that the Death Eaters from Harry Potter somehow make a pact with the vampires in Owari no Seraph. This forces the good guys from both universes, like Harry and Yuu, to join forces. They might have to figure out how to break the pact and defeat their common enemies. It could also involve some interesting character dynamics, like Draco Malfoy being forced to work with Mikaela, a vampire from Owari no Seraph, and slowly changing his views.
One idea could be a 'what if' scenario where the main characters have different abilities from the start. For example, Yuichiro could have a hidden power related to healing instead of his usual combat skills. This would change the whole dynamic of his relationships and the battles they face.
A plot idea could be that Mika discovers a secret about Krul's past that changes his perception of her completely. This could lead to him trying to protect her from those who would use the secret against her.
Some 'owari no seraph fanfiction' stories might be about alternate endings. What if the main characters made different choices? For instance, if Yuichiro decided to fully embrace his vampire side instead of fighting against it, how would that change the entire story? Writers can take this concept and create really engaging and thought - provoking fanfictions.
One great Naruto and Owari no Seraph fanfiction could be one where the characters from both universes team up to fight a common enemy. For example, Naruto's ninja skills combined with the vampire - fighting abilities in Owari no Seraph could create an exciting plot. Another might be a story where the main characters from each series meet and exchange their cultures and powers. There could also be a fanfic where a character from Naruto gets transported to the Owari no Seraph world and has to adapt to survive.
Naruto's ability to create and manipulate chakra could enhance the physical abilities of Owari no Seraph characters. For instance, if a character like Yuichiro had chakra, he could use it to boost his already fast and strong attacks. His cursed gear might also be able to be powered up with chakra. In reverse, the unique powers of Owari no Seraph characters could be added to Naruto's arsenal. Maybe Naruto could learn to use some of the vampire - like powers in combat.
Another idea is a crossover with a superhero series. The seraphs with their powers could team up with superheroes. They could work together to fight a common enemy that is threatening both their worlds. This would bring in a mix of superpowers and different combat styles.
One common theme is the clash of different power systems. In Naruto, there are ninjutsu and chakra, while in Owari no Seraph there are vampire powers and cursed gear. Fanfictions often explore how these powers interact when the two universes collide. Another theme is friendship across different worlds. Characters from Naruto and Owari no Seraph have to learn to trust and cooperate with each other.
One popular Naruto and Owari no Seraph fanfiction could be 'Crossover of Worlds'. It often has interesting character interactions where Naruto characters meet those from Owari no Seraph in a unique plotline. Another one might be 'Alliance in the Shadows' which focuses on how the two groups from different universes form an alliance to face a common enemy. There are also some that explore the power dynamics when characters from both franchises exchange their abilities.