Voldemort himself could be the main antagonist in this scenario. He might offer Hermione something she desperately wants in exchange for her betrayal. Maybe he promises to spare her Muggle - born friends if she gives up the Order's plans and secrets. His powerful and intimidating presence could easily force Hermione into a difficult situation where she feels she has no choice but to betray.
A new and powerful dark wizard or witch created specifically for the fanfiction could be the main antagonist. This character could have some sort of connection to Hermione's past or her family, and use that knowledge to force her to turn against the Order. They could be extremely cunning and have abilities that even the Order finds difficult to counter, making Hermione's situation even more desperate.
It could be a new character introduced in the fanfiction. Maybe someone who has infiltrated the Order and wants to create chaos by turning them against Hermione. This person could be a spy from the Death Eaters or another dark - aligned group, with the goal of weakening the Order from within by making them lose one of their most intelligent and powerful members.
In such a fanfiction, perhaps Hermione could be blackmailed into betraying the Order. Maybe an enemy discovers a deep secret about her family and forces her to give up Order secrets.
It could be a new villain that we haven't seen in the original Marvel universe. This villain might be able to manipulate Natasha's emotions and loyalty, making her betray Tony. For example, a powerful sorcerer who uses dark magic to control her.
It depends on the specific plot of the fanfiction. It could be a friend who turns against her due to jealousy, or perhaps a false ally within the so - called 'light' side who has ulterior motives.
It could be Lex Luthor. He's always looking for ways to take down superheroes. He might manipulate Wonder Woman into believing Batman is a threat, so she betrays him. Luthor is smart enough to create a complex scheme to drive a wedge between them.
If Rias betrays Naruto, she herself could potentially be the main antagonist. Since Naruto is usually seen as a protagonist, Rias' actions of betrayal would put her in direct opposition to him. Her motives and actions would then drive the conflict in the story.
The Konoha elders could be the main antagonists. They are often shown to be quite traditional and set in their ways in the original series. If Naruto goes against their old - fashioned ideas about the village's future and ninja code, they would likely oppose him strongly and could be the driving force behind making Naruto an outcast, which in turn could lead to his 'betrayal'.
In many fanfictions with such a plot, it could be a so - called friend who betrays Hermione perhaps out of jealousy. Maybe they are envious of Hermione's intelligence and popularity at Hogwarts, and for some self - serving reasons, they choose to betray her.
Perhaps a powerful magical being from a different realm is the antagonist. This being might mistake Hermione for someone else or see her as a key to something in the wizarding world. So, it uses its magic to make her disappear. This being would be very difficult to defeat or negotiate with, making it a huge challenge for those trying to find Hermione.
Another possibility is a dark wizard or a group of dark wizards who are after some powerful magic that the Lestrange family has, and Hermione gets in their way as she tries to protect the family or uncover the secrets. They would be the main force that Hermione has to face and overcome throughout the fanfiction.