Cedric Diggory could be an unexpected character in a betrayal scenario in fanfiction. Maybe in an alternate version, he becomes jealous of Harry's fame from being in the Triwizard Tournament and betrays him. This could be a really interesting twist as Cedric is generally seen as a good and honorable character in the original story.
Barty Crouch Jr., who was already a key figure in the original Goblet of Fire plot, could be further explored in fanfiction in terms of betrayal. What if there were more people involved in his scheme? What if he had accomplices within Hogwarts who were willing to betray the school for Voldemort's cause? This could lead to a much more convoluted and engaging story.
Snape is a likely candidate. In the original series, his loyalties were always a bit of a mystery at first. In fanfiction, he could be further explored in terms of false betrayals or real ones. Another one could be Draco Malfoy. He was on the verge of becoming a full - blown Death Eater in the original, so in fanfiction, he could either truly betray his Hogwarts classmates or be wrongly accused of betrayal.
Peter Pettigrew is a very likely character to be involved in more acts of betrayal in fanfictions. Since he already betrayed James and Lily Potter in the original story, it's easy to imagine him continuing his traitorous ways. Another could be Draco Malfoy. In some fanfictions, he might be made to do more extreme acts of betrayal rather than just being a bully. He could be forced to betray his schoolmates in a more serious way for the sake of his family's safety.
One possible plotline could be a friend betraying Harry to gain favor with the Death Eaters. Maybe they leak information about Harry's tasks in the Triwizard Tournament. This would add a whole new layer of danger and drama to the story.
Naruto himself could be a victim of family betrayal. His father Minato or mother Kushina might be involved in some alternate - universe plot where they turn against him for some dark reason. Sasuke's family is also a likely candidate. Itachi's actions could be re - imagined in a way where his real motives are more about betrayal of the Uchiha family rather than protecting it. And Hinata's family might be involved too, like if Neji, before his death, was made to betray the Hyuga family in some fan - created scenario.
Harry Potter himself is often involved. His inheritance from his parents, like the Invisibility Cloak and his family's magical abilities, can be a central part of the story. There could be situations where others suspect he might use his inheritances in a way that betrays the wizarding world, like if he was somehow influenced by the dark side of his family's magic.
Well, Ares could be involved. He has had his clashes with Percy in the original stories. In fanfiction, his war - like nature and desire for chaos could make him set up Percy to look like he's betrayed the community. Also, Octavian from the Roman side might be a character in such a scenario. He's power - hungry and jealous of Percy's status. He could spread false rumors or create situations that make it seem as if Percy has betrayed the Roman demigod community.
The village elders are often depicted as the ones involved in Naruto's betrayal. They might see him as a threat to their power or the stability of the village. Also, some fanfictions might involve Danzo. He has always been a bit of a shadowy figure in the Naruto series, and his underhanded ways could easily be written into a plot where he is behind Naruto's false accusations and subsequent banishment.
Carlisle could be involved in a betrayal story. Maybe he is betrayed by one of his own family members who disagrees with his peaceful co - existence ideology. This could lead to a rift in the Cullen family and force Carlisle to make difficult decisions about how to deal with the betrayal.
One likely character could be Draco Malfoy. He's always been antagonistic towards Harry, and in the third year, he might find a new and more devious way to cause trouble for Harry. He could pretend to befriend Harry to get closer and then betray him by spreading false rumors or stealing something important from Harry.
Sasuke is a likely candidate in some fanfictions. Given their complex relationship throughout the original series, it's easy to imagine a scenario where Sasuke's thirst for power or some misinformation leads him to betray Naruto. Another could be Sakura. If she becomes jealous of Naruto's growing power or if she's manipulated by outside forces, she might turn against him.