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.
Well, a really cool plot could be that the Tailed Beasts in Naruto are somehow related to the Gates in Darker than Black. Maybe the power of the Tailed Beasts can be used to open or close the Gates, which has a huge impact on both universes. And the characters from both Naruto and Darker than Black have to work together to stop a greater evil that wants to use this connection for its own malicious purposes. This would involve a lot of action, mystery and character development as they learn to trust each other.
A possible plot could involve Sasuke as a Contractor. His power could be something related to his Sharingan, like being able to see the true form of Contractors and Dolls. This would put him in a unique position in the crossover. Also, the story could focus on how the characters from Naruto adapt to the much darker and morally ambiguous world of Darker than Black. It could be a journey of self - discovery for them as they face new challenges and enemies that are not as straightforward as the ones in their own universe.
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 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.
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.
I think Naruto would be the most interesting. His positive and determined personality would contrast greatly with the darker and more cynical world of Darker than Black. He would be constantly trying to find the good in people and situations, which could lead to some really interesting character interactions. Also, his powerful jutsu and unique abilities would make him a force to be reckoned with in the crossover.
One great 'Darker than Black - HEI Crossover' fanfiction is 'The Unlikely Alliance'. It brings together characters from different arcs in a really interesting way. The author does a good job of maintaining the personalities of the characters from the original series.
The combination of different story elements from the 'Darker than Black' universe. We get to see HEI in new situations and interacting with various characters he might not have met in the original series.
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.