It could be a powerful and evil ninja from a rival village who has killed one of Boruto's close friends or family members.
In some fanfic scenarios, it could be a corrupted shinobi within the Hidden Leaf Village itself. This shinobi could have betrayed the village in some way that directly affects Boruto or his loved ones, like stealing important secrets that lead to the endangerment of the village. And Boruto, being the determined and loyal person he is, would seek revenge against this traitor.
It could be a rogue ninja who has a personal vendetta against Boruto's family. This ninja might have caused harm to Boruto's father Naruto in the past, and now Boruto is determined to make him pay.
In a 'Boruto Seeks Revenge Fanfic', Boruto might seek revenge against an enemy who has wronged his family. For example, if a new villain attacks Konoha and harms his father Naruto. Boruto could train intensively to gain power and then set out on a journey to find and defeat this foe.
Well, maybe Boruto's family or friends are hurt by a powerful enemy. So he decides to train hard and seek revenge. He might travel to different places to learn new jutsu and gather allies along the way.
Perhaps the target is Howard. Howard might have made a joke about Sheldon's mother that Sheldon didn't appreciate. As a result, Sheldon could try to get revenge by interfering with Howard's work on the space station projects, causing minor but annoying setbacks.
It could be the Kara organization. They've been shown to be interested in Boruto's Karma mark in the original series, so they might have finally made a move to abduct him in the fanfic.
Sarada could also be a main character. She is strong - willed and intelligent. She could be investigating a case of a missing - nin who is suspected of being involved in some sort of conspiracy against the Uchiha clan. This would give her a chance to show her detective skills and her loyalty to her clan.
It could be a rogue ninja who wants to use Boruto to get to Naruto. Maybe someone who has a grudge against the Hokage.
It could be a resurrected Orochimaru. He has always had an interest in power and the Akatsuki. He might return, more powerful than ever, and take control of the Akatsuki in the Boruto era, using them to carry out his evil plans, like creating an army of super - ninjas using some dark jutsu.
In a 'naruto dies boruto fanfic', Boruto's character would go through a major transformation. He would be forced to grow up overnight. Initially, he might be consumed by anger and a desire for revenge if Naruto was killed unjustly. But as time passes, he would learn to channel that energy into positive change. He would study his father's leadership style and try to implement it in his own way. He would become more diplomatic, forging alliances with other villages to strengthen Konoha's position. His sense of justice would be refined, and he would be more committed to protecting the world his father loved so much.
One possible plot could be that Boruto is on a dangerous mission to protect the Hidden Leaf from a new and powerful enemy. He sacrifices himself to save his friends and the village. Maybe he uses a powerful jutsu that drains his life force but defeats the foe.