Maybe a rogue ninja who has learned about modern technology and uses it to his advantage. He could be stealing valuable ninja artifacts and using modern gadgets to cover his tracks. He's smart like the villains in Case Closed, but also has the power of a ninja. He would be a difficult opponent for Naruto and Conan to face as he can blend the two different worlds' tactics.
It's possible that the main theme of 'behind closed doors novel' is about the contrast between public and private lives. The novel might show how characters present themselves to the outside world and what really goes on in their private, behind - closed - doors moments. It could also touch on themes of mystery, as what happens behind closed doors is often a mystery to others.
It's a graphic novel related to the 'Case Closed' series. Usually, it involves detective stories, with the main character solving various mysteries. It might have cool illustrations to go along with the exciting plots.
One popular 'Case Closed' fanfiction might be those that explore the relationship between Conan and Ran in different scenarios. For example, some stories could focus on how they might get closer to revealing Conan's true identity in a more romantic context.
A 'case closed age reversed fanfic' is likely a piece of fan fiction within the 'Case Closed' universe. By reversing the ages, it can completely change the dynamics. For example, relationships between characters might be different. It could also show how the characters would act if they were at different stages of life, adding new perspectives to the original story.
I'm not entirely sure as it could be a very specific or niche story. It might be about a love story that takes place in a metropolis in the context of a 'case closed' situation, perhaps a mystery or problem that gets resolved while the love story unfolds.