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.
A fanfic could also create a completely new villain for Kojou to face. This villain could have a unique power set that forces Kojou to develop new skills or find new allies. It could be a villain from a different dimension who has set their sights on Itogami Island for some mysterious reason, like harvesting its unique magical energy for their own dark purposes.
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.
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.
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.
One idea could be a fanfic that focuses on Cormoran Strike's early days in the military and how those experiences shaped his detective skills. Another could be a story where Robin and Strike have to solve a case in a foreign country, dealing with different cultures and languages. A third idea might be a fanfic that delves into Strike's family history and how it affects his present-day actions.
Well, you could write a fanfic that focuses on the backstories of the characters in 'Blood of Olympus'. Maybe explore how Percy Jackson became the hero he is from a different perspective. Or create a plot where the gods interfere more directly in the demigods' quests in the fanfic. This could add more drama and tension as the demigods have to deal with the gods' complex personalities and motives on top of their existing challenges.
Perhaps a fanfic where Naruto's blood has some special connection to an ancient and powerful demon. The demon is slowly being awakened due to some dark magic related to Naruto's blood. Naruto has to face not only the threat of the demon but also the distrust from some in the village who think his blood is a danger. He must prove that he can control this situation and keep the village safe.
One idea could be exploring the backstories of the characters more deeply. Maybe delving into their childhoods and how those experiences shaped them into the people they are in the story. Another could be creating an alternate universe where the main characters have different occupations or abilities.
In a 'dark blood Naruto fanfic', the dark blood could give the wielder the ability to see the darkest secrets of others. Naruto, with his new power, uncovers a conspiracy within the ninja world that is far more dangerous than anyone thought. He has to use this power, which is feared by many, to save the world while also dealing with those who want to stop him because of his 'dark' ability.