Perhaps in the fanfiction, an evil version of Goku from an alternate timeline arrives. This impostor causes chaos and destruction, but everyone blames the real Goku. His family and friends are deceived by the look - alike. Goku tries to prove his innocence but no one will listen. He might have to find a way to expose the impostor and regain the trust of those he loves.
In the 'goku and gohan betrayed fanfiction', perhaps they are betrayed due to a misunderstanding. Say, Goku makes a deal with an unknown force to save the universe, but it's misconstrued as him allying with the enemy. Gohan, being associated with Goku, is also seen as a traitor. And this misunderstanding could be spread by a character with their own motives, like a power - hungry individual who wants to take over the Z - fighters' leadership.
Well, one possibility is that Naruto is framed for a serious crime he didn't commit. Maybe someone fakes evidence that he's been working with the enemies of the village. His friends, believing the false evidence, turn their backs on him. He then has to fight not only against the village's distrust but also to clear his name and find out who really set him up.
In a Naruto betrayed by everyone harem fanfiction, the plot might start with Naruto being framed for a major crime against the village. The harem elements could be that his so - called harem members, who are usually his friends and admirers, are caught in a dilemma. They love Naruto but the evidence against him seems overwhelming. As the story unfolds, Naruto may have to fight not only to clear his name but also to win back the trust of his harem. It could involve him going on a journey to find the real culprit while facing the animosity of his former friends and the village he once loved.
Well, perhaps in this fanfic, Harry could be wrongly accused of a serious crime like dark magic use. Everyone, including his friends Hermione and Ron, might turn their backs on him due to false evidence. He would then have to fight alone to clear his name and find out who really framed him.
Maybe it's a new character created specifically for the fanfiction. This character could be a powerful sorcerer who has the ability to manipulate people's minds. He targets Goku out of jealousy for his power and popularity. By using his mind - control powers, he makes everyone around Goku believe that he's a traitor, thus making himself the main antagonist.
One of the characters who could betray Goku is Piccolo. Although they've been allies for a long time, in fanfiction, things can change. Perhaps an ancient evil force takes control of Piccolo's mind and makes him turn on Goku. Or it could be Krillin. He might be influenced by an outside force that convinces him Goku is becoming too dangerous. Also, don't forget about the Kais. If there's some sort of power imbalance or a threat they perceive from Goku, they might betray him to maintain the order of the universe in the fanfiction.
Another possibility is that there is some kind of dark influence at work. A powerful and malevolent force could be manipulating people's minds to turn them against Ash. This force might see Ash as a threat and use this underhanded method to get rid of him, causing his world to crumble as he is betrayed by all.
In a fanfiction like this, it could be that Percy's powers or abilities were starting to make others jealous. His friends might have felt overshadowed and, in a moment of weakness, turned on him. Also, there could be an outside influence, like a dark force that was manipulating people's minds to make them betray Percy.
One way this could develop is if Naruto's identity as the Jinchuriki of the Nine - Tails is fully exploited against him. People might fear his power and turn on him. For example, the village elders could spread false rumors about him losing control of the Nine - Tails, causing the villagers to shun him. Another possibility is that a powerful enemy uses mind - control or deception to make those close to Naruto, like Sakura or Kakashi, turn against him. This would be a huge blow to Naruto as he values his relationships deeply.
It could be Vegeta. Maybe in this fanfiction, his pride gets the better of him and he feels overshadowed by Goku and Gohan's power. So he fabricates some lies to make them look like traitors to the rest of the group.