One possible plot could be that Mega Man is betrayed by his creator. Maybe the creator decides Mega Man has become too powerful and tries to shut him down, leading Mega Man to fight for his own survival while feeling deeply hurt by the betrayal.
One possible plot could be that Tails is betrayed by a close friend who becomes jealous of his intelligence and skills. For example, Sonic might start to feel overshadowed by Tails and in a moment of weakness, turn against him. This could lead to Tails having to fend for himself and prove his worth all over again.
One possible plot could be that Boruto is betrayed by his friends due to a misunderstanding. For example, they might think he has turned to the dark side because of some false evidence planted by an enemy. Another could be that Boruto is betrayed by the village he loves. Maybe the village leaders suspect him of a crime he didn't commit and turn against him.
One possible plot could be that Fleur betrays Harry in a Triwizard Tournament - related task, perhaps by secretly helping the other champions. Another might be that Harry discovers Fleur has been passing information about the Order of the Phoenix to the wrong side. Maybe Fleur betrays Harry's trust regarding a personal secret, like his feelings for Ginny, and uses it to her advantage in some way.
In some 'Wei Wuxian betrayed fanfiction', it might be that he is betrayed by a family member or a disciple he had trained. For example, they might side with his enemies in a battle, believing the lies told about Wei Wuxian's so - called 'evil ways'. This would make Wei Wuxian feel extremely hurt and isolated, having to face the challenges alone.
One possible plot could be that Ash discovers Anabel has betrayed him by siding with a rival trainer in a crucial tournament. This could lead to a rift in their friendship and Ash having to find a way to overcome this setback and prove himself without her support.
One possible plot could be that everyone believes false rumors about Harry being a dark wizard. His friends from Hogwarts, including Ron and Hermione, turn their backs on him. Harry then has to find a way to prove his innocence all alone, perhaps by going into hiding and secretly investigating who spread the lies.
One possible plot could be that Ash is falsely accused of a serious crime within the Pokémon world, and Greninja, due to some misinformation or outside influence, turns against him. Maybe a powerful villain manipulates Greninja's mind, making it believe that Ash is a threat to the Pokémon world. This could lead to intense battles between them, with Ash being heartbroken and trying to prove his innocence while also trying to save Greninja from the villain's control.
Perhaps Ash is wrongly accused of something in Skyla's hometown. Skyla believes the false accusations and turns her back on Ash, which makes Ash feel extremely betrayed. He then has to prove his innocence not only to Skyla but also to the whole town.
One possible plot could be that Naruto's family, perhaps his father or mother, were actually alive but had to fake their deaths for some dark reason. Then they reappear and turn against Naruto, maybe because they are being controlled by an evil force. Another plot might be that his family had some long - hidden secrets that made them see Naruto as a threat to their real goals, so they betrayed him.
Perhaps in the fanfiction, Natsu is betrayed by someone he trusts in a mission. For example, they might leave him behind in a dangerous situation to fend for himself while they take the credit for the mission. Or it could be that a love interest betrays Natsu by choosing someone else over him due to some false perception or external pressure.