Who could be the main villain in a 'Harry Disappears' fanfiction?

A secret society that has been observing the Avatar from the shadows could be the main villain. They believe that Aang's power is too great and unpredictable, so they use some ancient and powerful artifacts to make him disappear. The heroes would then have to figure out how this society operates and find a way to reverse the spell or device that made Aang vanish.

Orochimaru could be a likely main villain. He has always been interested in Naruto's power and body. Maybe he finally devises a plan to kidnap Naruto and use his body for his own reincarnation experiments.

Voldemort could be the antagonist. Even if he was defeated in the original story, there could be a scenario where some remnant of his dark magic causes Harry and Hermione to disappear. Maybe he had a secret plan that was set to activate after his death and it involved taking them out of the picture.

The Queen of Hearts would make a great main villain. Her tyrannical rule and quick temper in the original Wonderland story could be amplified in a fanfiction. She could be a powerful dark magic user who sees Harry as a threat to her rule and tries to stop him from achieving his goals in Wonderland.

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.

It could be Dumbledore. Even if he's dead in the original story, in fanfiction, anything is possible. He might have left behind some kind of magic that allows him to train Harry from beyond the grave, or he could have faked his death to take Harry to a secret location for training.

Maybe Voldemort's old followers who are still up to no good. They might think that by getting Harry to kidnap Muggles, they can bring back the dark days under Voldemort's rule. They could be using some sort of dark magic to influence Harry's actions.

It could be a rogue member of the Order. Maybe someone who has their own agenda and decides that Harry is a threat to their plans. So they manipulate the Order into kidnapping Harry, but then they have their own evil intentions for him.

Maybe Thanos somehow discovers the wizarding world and decides he wants to add the power of magic to his collection of Infinity Stones. He could be the main villain, and Harry, with his understanding of both magic and his new Avengers - like abilities, would have to lead the fight against Thanos to protect both worlds.

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.