A great plot idea would be a tournament where characters from both Naruto and Strike the Blood participate. Naruto's friends like Sasuke and Sakura could also be brought into this new world. They would have to learn the rules of the Strike the Blood world quickly and use their ninja skills to compete. The vampires in Strike the Blood might be intrigued by the ninja powers and try to learn from them. This could lead to some interesting alliances and rivalries.
A third plot idea is a character - swap scenario. What if Kojou Akatsuki wakes up in the Naruto world with his vampire powers? He would have to navigate the world of ninjas, clans, and hidden villages. Meanwhile, Naruto could end up in the Strike the Blood world, having to deal with the complex politics between the different vampire factions while also trying to find a way back home.
A great plot for Strike the Blood fanfiction could be a time - travel scenario. Kojou is sent back in time to a period when vampires were first emerging. He has to use his modern - day knowledge and powers to survive in this ancient and dangerous time, while also trying to find a way back to his own time. This would allow for a lot of world - building and exploration of the vampire lore in the series.
One idea could be an OC who is a powerful vampire hunter sent to the island where the story takes place. They initially clash with the main characters but end up teaming up to face a greater threat. For example, a new type of ancient vampire that is immune to the usual methods of slaying.
One idea could be that Naruto characters are transported to the Trinity Blood universe. Naruto, with his strong will and powerful chakra, might be seen as a threat or a potential ally in the complex political and religious landscape of Trinity Blood. For example, his shadow clone jutsu could be mistaken for some form of dark magic in that world.
Another plot could focus on the origin stories of some of the side characters. For instance, the backstory of Yukina Himeragi. How did she become an attack mage? What kind of training did she go through? A fanfic could expand on this and also show how her past experiences influence her actions and relationships in the present story.
One idea could be that the Spartans from Halo and the Witches from Strike Witches team up to fight a common alien threat that has invaded both of their universes. The Spartans' advanced weaponry and combat skills combined with the Witches' magical abilities would make for an epic battle.
One idea could be that they get separated due to a powerful enemy's scheme. Kojou tries desperately to find Yukina while she, on the other hand, is fighting her own battles to get back to him. Their love for each other gives them the strength to persevere through all the hardships.
In a Naruto and Strike the Blood fanfiction, the characters' personalities would play a huge role. Naruto's determination and never - give - up attitude could inspire the characters from Strike the Blood. And the more complex and often morally ambiguous characters from Strike the Blood could introduce a new level of depth to Naruto's world. Maybe they could even teach Naruto some new ways of looking at power and responsibility.
One possible plot could be that the ninjas from Naruto world discover the existence of the Strike Witches. They could initially be in conflict due to differences in their fighting styles and goals. But then, they realize they have a common enemy, like an ancient dark force that threatens both their worlds. They then combine their powers, with the Naruto characters using their jutsus and the Strike Witches using their magic and flight abilities to fight this new threat.
Perhaps a love triangle could be created in the crossover fanfiction. A female character from 'Twilight' could fall in love with a vampire from 'True Blood' while also having feelings for a character from her own world. This would create a complex web of relationships and loyalties, and the different vampire abilities and cultures from both series would add depth to the story.