It could be really strange. Maybe Harry Potter, instead of being a wizard, is a White Walker. He might have the ability to raise the dead in the wizarding world, which would cause chaos. His friends would be horrified and they would have to find a way to stop him or bring him back to his normal self.
Well, it could be an epic and very different kind of fanfiction. The White Walker lore is full of mystery and danger. If Harry were a White Walker, his connection to the magical world might be severed or transformed. He could bring a new form of magic, one that is cold and death - based, to the wizarding world. For example, instead of using a wand to cast spells, he might use his icy touch. And the story could explore how the other characters react to this. Some might fear him and try to exile him, while others might see him as a potential weapon against a new, greater evil. It would be a story that combines the best elements of both universes while creating a unique and often disturbing narrative.
A 'Harry Potter is a White Walker' fanfiction could be a dark and thrilling story. Picture Harry with the pale blue eyes of a White Walker and the power to freeze things. He could turn Hogwarts into an icy fortress. The other characters would have to adapt quickly. Hermione would be researching like mad to find a way to reverse the transformation, while Ron would be trying to fight Harry with his limited skills. The story could also involve the White Walker army invading the wizarding world, and the wizards and witches having to unite against this new threat in ways they never thought possible. This would completely change the dynamics of the Harry Potter universe and create a whole new set of challenges for the characters.