A great idea for the fanfiction could be to introduce a new, extremely powerful and malevolent villain. This villain could be so powerful that it forces Naruto and his friends to use unorthodox and morally questionable tactics. The villain could also have the ability to corrupt the minds of the villagers, creating a sense of paranoia and distrust among everyone in Konoha.
Another interesting aspect could be delving into the backstories of minor characters but in a much darker light. For instance, what if Rock Lee had a past filled with abuse and trauma that made his drive to succeed even more complex? In this 'Naruto darker than black' fanfiction, we could see how these darker backstories shape the characters' present actions.
One plot idea could be that Naruto characters gain the powers of Contractors from Darker than Black. For example, Naruto himself could have a Contractor power related to his shadow clones, like being able to send his clones to different dimensions for a short time. Another idea is a story where the organizations in both universes collide. The Akatsuki from Naruto and the Syndicate from Darker than Black might have different goals but similar methods of achieving them. They could either fight or try to make an alliance depending on the situation.
One idea could be exploring Hei's past in more detail. Maybe a story where he has to face a threat from his old organization that forces him to remember things he'd rather forget. Another could be a fanfiction where Hei teams up with an unexpected ally, like a character from a different anime universe, to solve a mystery in the Darker than Black world.
One idea could be a story where Hei has to protect Misaki from a new, powerful enemy that is after her because of her knowledge of Contractors. This would put them in a lot of dangerous situations and force them to rely on each other more than ever.
A plot idea could be that there is a strange phenomenon affecting Contractors and Dolls, and Misaki and Hei are the only ones who seem to be immune at first. They then have to investigate the cause while also dealing with the suspicion and fear from others in their respective communities. As they dig deeper, they find out that it has something to do with an ancient and forgotten power that is starting to resurface.
One interesting story could be about a new Contractor with a unique power that changes the power balance in the Darker than Black world. Maybe they have the ability to manipulate time on a small scale, which leads to all sorts of adventures and conflicts with other Contractors.
One interesting crossover could be a scenario where characters from Naruto enter the world of Darker than Black. For example, Naruto's ability to never give up and his positive attitude might contrast with the often dark and morally ambiguous world of Darker than Black. He could potentially influence some of the Contractors there to question their own motives and actions.
Perhaps a plot where an outside force tries to manipulate Hei and Yin's relationship. They have to figure out who is behind it and in the process, they learn more about their own feelings for each other. This could involve some intense action scenes as they are constantly under attack while trying to solve the mystery.
One plot idea could be an OC who discovers a hidden conspiracy within the Syndicate. They start to uncover secrets that could change the balance of power in the Darker than Black world. Another idea is an OC Contractor who has lost their memory and is trying to figure out their past while being hunted by other Contractors. Or, a plot where an OC is a normal human who gets caught in the middle of a war between Contractors and tries to find a way to bring peace.
The mystery surrounding November 11 in Darker than Black provides a rich ground for fanfiction. It has an air of enigma that writers can explore further, like what really happened on that day, what secrets are hidden, etc.
The complex relationship between Amber and Hei makes it interesting. Their personalities are so different yet they are intertwined in the story.