One interesting plotline could be the origin of the black blade. Maybe it's a weapon created by an ancient shinobi clan with a special power. For example, it could be a blade that absorbs the chakra of those it cuts, making the wielder stronger.
Perhaps in the 'black blade naruto fanfic', the black blade has a connection to an ancient and forgotten clan. Naruto, being the main character, then delves into the history of this clan, uncovers secrets, and uses the knowledge and power associated with the blade to protect the Hidden Leaf Village from a looming threat that only he with the black blade can face.
One possible plotline could be how the characters in Naruto deal with the sudden outbreak of a Black Death - like disease. Maybe Naruto himself tries to find a cure using his shadow clones to search far and wide.
One interesting plotline could be the main character's journey to discover their true heritage within the world of 'the black heir fanfiction'. They might face challenges and make new alliances along the way.
One interesting plotline could be the mystery behind the mask. Maybe it has some special power that only Naruto can unlock or it could be related to an ancient prophecy. Another could be a new villain who uses the mask to create chaos in the ninja world and Naruto has to stop him.
Perhaps Naruto discovers a new medical jutsu that combines his chakra control with traditional medical knowledge. This could revolutionize the medical field in the Naruto universe. He might use it to heal previously incurable diseases or injuries, and in the process, gain the respect of the medical community. This could also lead to some interesting interactions with other medical ninja, like Shizune or Tsunade.
Well, in some fanfics, there's a plotline where Hinata loses her memory and Naruto has to make her fall in love with him all over again. It's a classic but still very engaging. There are also stories where they are arranged to be married for political reasons at first, but then they actually fall in love for real during the process. And a really unique one is when Hinata becomes a super powerful ninja overnight and Naruto has to deal with his feelings of being a bit overshadowed while also being proud of her.
One interesting plotline could be Naruto joining the Akatsuki and trying to change it from the inside. He might use his ideals of friendship and peace to influence the other members who have mostly known a life of darkness and violence.
One interesting plotline could be Shizune's involvement in secret medical research within the Hidden Leaf. Maybe she discovers a new form of medical ninjutsu that could change the way they treat serious injuries. Another could be her relationship with Naruto from a medical - training perspective. She helps him understand the importance of medical skills for a ninja, and they bond over that.
One plotline could be Scathach training Naruto in some ancient and powerful techniques that are not known in the ninja world. This would give Naruto an edge over his enemies and also lead to interesting interactions between them as Naruto tries to master these new skills.
One interesting plotline could be Naruto dealing with the complex political landscape of the Hidden Leaf as an adult. He might have to make tough decisions regarding alliances and the future of the village.