Maybe it's a new character created by the fanfiction author. This witch could have some dark magic tendencies or a past that Dumbledore found unforgivable. For instance, she might have been involved in some unethical magical experiments in the past that Dumbledore strongly opposed.
The marriage could also have an impact on Harry's friends. They might be caught in the middle between their loyalty to Harry and their respect for Dumbledore. For example, Ron and Hermione might struggle to support Harry wholeheartedly if Dumbledore makes his disapproval clear. Moreover, the witch herself would likely face a great deal of hostility from Dumbledore's supporters, which could make her and Harry's life together quite difficult.
Perhaps a witch who has lost her family to witch hunters decides to take matters into her own hands. She disguises herself as a Muggle and tries to infiltrate the witch hunter's group to take revenge. This would make her a very interesting and complex main character.
One reason could be secrecy. If Harry finds out that Dumbledore has been keeping major secrets from him all along, like about his family or his true role in the wizarding world, he might start to hate him. Another reason could be manipulation. If it seems that Dumbledore has been manipulating Harry's actions for his own gain, that would be a valid cause for hatred in fanfiction.
It could be that Harry finds out about Dumbledore's secrets regarding the Deathly Hallows. If Harry thinks Dumbledore was more interested in the Hallows for his own purposes rather than for the greater good, it might make him hate Dumbledore.
Perhaps a new type of creature that is neither from the Harry Potter world nor the Witch Hunter's world. It could be a result of some dark magic experiment gone wrong. This creature could have the ability to nullify magic in some ways, which would be a huge threat to both Harry and the Witch Hunter.
In Harry Potter fanfiction, the 'the witch that time forgot' could be a self - made character by the fanfic writer. She could be a witch from an ancient era who was shunned or lost in time. Maybe she was a victim of a dark wizard's curse long ago that made her disappear from the memories of the wizarding world. And now, she resurfaces in the fan - created story with a vengeance or a mission to reclaim her place or seek revenge for what happened to her in the past. She could also be a misfit witch who was never really part of the mainstream wizarding society and was thus forgotten as time passed by.
The theme of disillusionment also often appears. Harry, who once idolized Dumbledore, now sees him in a new light. He realizes that Dumbledore is not the perfect mentor he once thought, and this realization leads to his hatred. This can be explored in various ways, such as through flashbacks or interactions with other characters who have different views of Dumbledore.
Yes, there might be some out there. Fanfiction is a vast and diverse world where writers explore all kinds of alternative scenarios. Some writers may create stories where Harry Potter hates Dumbledore for various reasons, such as feeling manipulated or betrayed by Dumbledore's actions. However, this goes against the general relationship portrayed in the original books, which is why it's an interesting and somewhat controversial topic in fanfiction.
One possibility is that Petunia could use her witch powers to protect Harry secretly. Maybe she has a change of heart after years of resentment. She could have been in contact with the wizarding world all along, but hid it from everyone. This would add a whole new layer of mystery and family drama to the Harry Potter universe.
The main character could be an orphan with no knowledge of magic at first. They are just a normal kid until they are adopted by the witch. The witch herself would be another main character. She could be an old, wise woman with a kind heart but also strict when it comes to teaching magic. There could also be a magical pet, like a talking cat, that serves as a sidekick to both the main character and the witch.