If Petunia was a witch, it would completely change Harry's upbringing. He might have learned magic from her instead of being in the dark until he received his Hogwarts letter. This would make him more confident and perhaps less of an outcast in the Muggle world.
The wizarding world's view of the Dursleys would be different too. They might see Petunia as an important figure, especially if she had some unique magical abilities. This could lead to the Dursleys being more involved in the wizarding world, whether they liked it or not. And it could also mean that Harry had more allies and resources within his own family before going to Hogwarts, which would have a significant impact on how he faced the challenges at Hogwarts and later on.
It would also change the relationship between Petunia and the Dursleys. Vernon might be even more afraid of her if he knew she was a witch. Petunia could use her magic to keep Vernon in line or to secretly make life better for Harry. The family dynamic would be much more complex and full of secrets. For example, she could use magic to make sure Dudley didn't bully Harry as much, but in a way that no one suspected magic was involved.
The plot could involve Petunia using her witch powers to protect Harry from the Dursleys' abuse. She realizes that she has a responsibility towards her sister's son and uses her magic to create a safe haven for him within the Dursley household. This would be a big change from her original character who was so against magic.
One possible fanfiction could be about Petunia discovering her latent witch powers later in life. Maybe she stumbles upon an old family heirloom that unlocks her magical abilities. This could lead to a whole new dynamic in her relationship with Harry, as she now understands the world he belongs to on a deeper level.
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.
Perhaps Aunt Petunia is a witch who was cursed long ago. The curse made her forget her magic and act like a Muggle. When Harry unknowingly breaks the curse, she suddenly has her powers back. She then has to face the wizarding world again and deal with all the changes that have occurred since she last remembered her true self. She might even have some unique powers that are different from the norm because of the curse.
One possible fanfiction could be about Aunt Petunia discovering her latent witch powers late in life. Maybe she starts having strange dreams that lead her to an old family heirloom that unlocks her magical abilities. She then has to deal with the shock of it all while also trying to keep it a secret from the Muggle world around her.
The plot would change a great deal. For one, Harry would be much more powerful physically. So when facing Voldemort, he could use Thor's strength in combat, not just rely on magic. He might be able to take on Death Eaters more easily.
His relationship with Hermione would become more complex. Hermione, being a firm believer in the rights of wizards and the value of magic, might be initially suspicious of Harry. She'd worry that his witch hunter heritage could cloud his judgment. However, as they've been through so much together, she'd also be one of the first to try and understand him. She might start researching about witch hunters to see if there's a way to reconcile Harry's two sides.
The plot would change significantly. For one, the final battles against the dark forces would be different. Instead of a wand - only battle at Hogwarts, there could be a full - scale war with modern weapons. The Death Eaters might not be prepared for the firepower of things like machine guns or grenades. It could also change the way characters interact. Hermione might study the mechanics of modern weapons instead of just ancient spells, and Ron could be in charge of weapon supplies.
In this fanfiction concept, the plot would see major alterations. With Flash's influence, Harry might not rely as much on traditional magic for transportation. He could zip around Hogwarts and the wizarding world at high speeds, changing the dynamics of his adventures. His battles against dark wizards would be different too. Instead of long, drawn - out duels, he could quickly close the distance and catch his opponents off - guard. Also, his relationship with Dumbledore might be different as he has this new, unorthodox way of dealing with threats.
The plot could change a great deal. For one, the threat of Voldemort might not be as immediate or known in the same way. Harry could be dealing with different dark forces in America, like some ancient and malevolent American - based dark wizards. His journey to discover his past and his power would likely take different turns as he navigates the American wizarding world.