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.
Well, perhaps in this fanfiction, the Black Death is caused by some dark jutsu gone wrong. And Sasuke might be involved in some way, either trying to stop it or being wrongly accused of causing it. He could use his Sharingan to investigate the source of the disease.
One interesting plotline could be Naruto having to team up with unexpected allies in the death race. Maybe he has to work with former enemies to survive the dangerous challenges of the race.
Perhaps a plot where Naruto makes a deal with the Death Reaper to save his friends. He has to go on a dangerous quest in the afterlife or something like that. It could involve facing his own fears and the dark secrets of the ninja world.
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.
One interesting plotline could be the exploration of new jutsus related to the Death God. Maybe the protagonist discovers a long - lost technique that the Death God once used. Another could be the interaction between Naruto and the spirit of the Death God, leading to some unforeseen consequences for the ninja world.
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 that Harry gains unique powers or abilities from being raised by Death. For example, he might have an enhanced understanding of life and death magic, which gives him an edge in battles against dark wizards. Another plotline could involve his relationship with other characters being drastically different. Since he's raised by Death, he might be more isolated from his peers at Hogwarts, leading to a complex web of friendships and rivalries.
Another plotline could be a death battle where a character from RWBY has to fight an alternate version of themselves. It would be a psychological and physical battle as they grapple with their own strengths and weaknesses in a different form.
One possible plotline could be that Harry discovers new magical abilities related to the concept of death. For example, he might be able to communicate with the spirits of the dead in a more profound way than just using the Resurrection Stone. Another could be that his friendship with 'death' gives him a unique perspective on life and sacrifice, leading him to make different choices in his battles against Voldemort.
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 possible plotline could be that Naruto faked his death to go undercover and investigate a powerful new enemy that was targeting him specifically. He might have known that his presence was putting the village in danger, so he faked his death to draw the enemy out.