Who could be the main villain in a fem Naruto and Harry Potter crossover fanfiction?

It could be a Death Eater who has somehow made a pact with a powerful force from Scorpian's world. This villain would be extremely dangerous as they'd combine the dark magic of the Harry Potter universe with the unknown and potentially deadly powers from Scorpian's realm.

It could be a wizard who has gone mad with power and wants to control both the wizarding world and the underwater realm. He discovers a way to use dark magic to manipulate sea creatures and cause destruction in Aquaman's kingdom. He then sets his sights on Hogwarts as well, seeing it as another source of power.

Voldemort could still be the main villain in a different way. Suppose he discovers Tonks' unique connection to Harry and tries to use her as a weapon against him. He could manipulate her mind or put her in dangerous situations to force Harry to make sacrifices or bad decisions.

Voldemort could be one of the main antagonists. He could follow the female Harry Potter through the portal to Middle - earth, causing chaos there. Also, the orcs and other dark creatures from Middle - earth like the Nazgûl could be antagonists. They would oppose the female Harry Potter and the hobbits or dwarves she teams up with.

Obviously, the fem Harry Potter would be one of the main characters. And perhaps she could meet Naruto in an anime world. Naruto's determination and her own magic skills would make them a great duo to face challenges together.

A powerful dementor - like being that has been corrupted by Asgardian dark magic. This entity could be draining the life force of both wizards and Asgardians. It would be up to Thor and Harry Potter to find a way to stop it, as it poses a threat to both of their worlds.

Kaguya could be a great main villain for this. Imagine Kaguya emerging on Halloween night, her already otherworldly appearance made even more menacing with spooky Halloween - like enhancements. She could use the energy of the night to her advantage, trying to plunge the world into eternal darkness, and Naruto and his friends have to stop her.

Lex Luthor from Supergirl's world could be the villain. He could somehow find a way to travel to the wizarding world and team up with some dark wizards. He wants to use magic to enhance his power and take over both worlds. Supergirl and Harry Potter would have to stop his evil plans.

One possible plot could be a fem Naruto being somehow transported to the Harry Potter world. She might initially struggle to understand the magic system there as it's so different from chakra. But her ninja skills could come in handy, like her stealth and combat abilities. For example, she could help Harry and his friends during a dangerous mission at Hogwarts by using her unique fighting style.

It could be a new character created by the fanfiction author. Maybe a powerful wizard who was once shunned by the wizarding community and now wants revenge by stealing magic.