One possible plotline could be a powerful new enemy emerging that neither Naruto nor Boruto can defeat alone. Maybe an ancient evil that has been awakened and has powers far beyond what the ninja world has ever faced. Their deaths could be a sacrifice to seal this evil away, protecting the village and the world at large.
It could also be a situation where a disease or a curse spreads throughout the ninja world. Naruto and Boruto, in their attempts to find a cure and save others, end up being affected by it. Despite their best efforts, they succumb to the illness or the curse, and their deaths become a symbol of hope for the future as the ninja world unites to fight against the common threat.
Some fanfictions might explore the idea of Naruto being betrayed by someone close to him. This betrayal could lead to a situation where he is caught off - guard and killed. It adds a dark and dramatic twist to the story.
Maybe Naruto's death powers are a result of some ancient curse or a powerful jutsu gone wrong. This could lead to a plot where he has to find a way to reverse it, while also using it to protect his village. He might encounter other shinobi who have similar powers and learn from them how to use it more effectively.
Maybe in the fanfic, Death is not a traditional Grim Reaper - like figure but rather a form of an evil curse that has been plaguing the ninja world. Naruto, being the hero, is the only one with enough positive energy and determination to face it. He could start by gathering allies who have also been affected by this 'Death' curse. Together, they form a plan. Naruto would be the vanguard, using his speed and strength to break through the defenses of the curse - based Death and then use his willpower to shatter it completely.
Perhaps in the fanfiction, Naruto gives his life energy to Boruto to save him from a deadly curse. Boruto then has to deal with the newfound power within him, which is a mix of his own and his father's. He may struggle to control it at first but eventually learns to use it for good. This could also lead to a rift between Boruto and some of the villagers who blame him for Naruto's death.
In the 'boruto kills naruto fanfiction', a motive could be a power struggle. Boruto might believe that Naruto's power is holding back the village's growth or his own potential. He could think that by eliminating Naruto, he can bring about a new era of power in the Hidden Leaf. Also, it could be out of desperation. If Boruto is in a situation where he believes that killing Naruto is the only way to save someone he loves, like his mother or a friend, he might take such a drastic step.
It could also be that Naruto sees in Boruto's eye a sign of an emerging danger. The eye might hold a clue to a new enemy or a brewing conflict. Naruto, being the experienced ninja he is, would then start to take precautions and train Boruto more intensively to face whatever this sign in his eye indicates.
Perhaps he has discovered a great threat that is specifically targeting him. To keep the village and his family from being collateral damage, he fakes his death. This way, he can secretly gather information, train more, and come back stronger to face the threat when the time is right.
Well, it could start with Hermione being kidnapped by the Death Eaters at a young age. She might be forced to attend their dark magic lessons. However, she uses her intelligence to learn about their weaknesses. For instance, she could discover a flaw in the security spells around their lair. And then, as she grows up, she could start to subtly manipulate the Death Eaters into fighting among themselves, all while pretending to be a loyal follower.
Maybe it's the high expectations that Naruto has for Boruto. Naruto is a hero, and he might unknowingly put a lot of pressure on Boruto to follow in his footsteps. Boruto could be scared of not being able to meet those expectations.
The plot could also involve a time - related crisis. Maybe something is threatening the timeline, and the only way to fix it is for future Naruto to raise his younger self in a certain way. So, future Naruto has to make sure that the younger Naruto grows up to be strong enough to face this threat while also maintaining the integrity of the timeline.