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.
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'.
An interesting plot could revolve around an OC who is an amnesiac. They wake up in the middle of the post - apocalyptic world of 'Seraph of the End' with no memory of who they are or where they came from. As they try to piece together their past, they discover that they have a special connection to both the humans and the vampires. Maybe they were part of an experiment in the past that gave them unique abilities. Their journey to recover their memory and understand their true nature could be full of twists and turns, and they might end up being the key to peace between the two warring factions.
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.
Kakashi and Guren could make for an interesting pair. Kakashi is a highly experienced and intelligent ninja, while Guren is a powerful vampire. Their different combat styles and knowledge could lead to some great interactions. Kakashi could try to figure out the secrets behind Guren's vampire powers, and Guren could be impressed by Kakashi's strategic mind and wide array of ninja techniques.
Maybe in the crossover, a strange curse or magic affects both worlds and starts to merge them. Naruto and his comrades find themselves in a world filled with both demons and vampires. Yuuichiro and his group are also confused but quickly realize they need to work together. The shinobi could use their shadow clone jutsu to create distractions during battles against the vampires, while the vampires' superhuman strength and speed could be an advantage in the open combat scenarios in the Naruto world.
The male OC could also be a scientist in this fanfiction. He is trying to find a way to end the war between vampires and humans through scientific means. He might be working in a secret laboratory, creating potions or devices that could potentially change the balance of power. He could have a sharp intellect and be a bit of an outcast among his peers.
You can start by creating a scenario where the two worlds collide. Maybe it's a portal that opens up between the 'Seraph of the End' world and the other world. Then, introduce the main characters from both sides. Show how they react to the new and strange elements in each other's worlds. You could start with a character from 'Seraph of the End' stumbling into the other world by accident.
One common theme is power struggle. Since 'seraph of the end' has a complex power system among vampires, humans, and seraphs, when crossed with other universes, there is often a new power struggle. For example, if crossed with a superhero - based universe, who is more powerful - a vampire or a superhero? Another theme is survival. Both 'seraph of the end' and many other fandoms often deal with post - apocalyptic or dangerous situations, so survival becomes a key theme in crossovers.
One interesting idea could be a crossover with "Moana". Elsa's ice powers and Moana's connection to the ocean could create a really cool dynamic. Maybe Elsa accidentally freezes part of Moana's ocean and they have to work together to fix it. Another could be with "Tangled". Rapunzel's long hair and her adventurous spirit combined with Elsa and Anna's journey in Frozen 2 would be exciting. Rapunzel could use her hair to help them climb mountains or cross chasms. A third idea is a crossover with "Beauty and the Beast". Belle's love for books and knowledge could help Elsa understand more about her powers and the history of magic in the Frozen world.
A Thundercats and Star Wars crossover fanfiction would be really cool. The Thundercats could be in the Star Wars universe, and their unique powers and weapons could be a great addition. Maybe the Thundercats could help the Jedi or the Rebels in their fight against the Sith or the Empire. For instance, Cheetara's speed could be useful in battles, and Tygra's illusions could be used to deceive the enemy.