Who could be the main antagonist in 'harry loses his twin fanfiction'?

It could be a new dark wizard who has a vendetta against Harry. This wizard might have created a spell specifically designed to target Harry's emotions, as he knows that Harry's strength often comes from his love and friendship. By taking away his emotions, he hopes to weaken Harry and make him easier to defeat.

It could be Voldemort, even if he's already been defeated in the original story. His lingering influence might still haunt Harry. Maybe there are some dark magic remnants of Voldemort that keep causing problems and making Harry feel hopeless.

It could be a new dark wizard who has a vendetta against Harry. This wizard might have specifically targeted Harry's leg as a way to cripple him both physically and mentally. Maybe this antagonist has some sort of connection to Voldemort's old followers, seeking to finish what Voldemort started.

Maybe in the fanfiction, Harry's twin could be lost in a magical accident. For example, during a wizarding battle, the chaos causes them to be separated, and Harry spends the rest of the story trying to find his twin.

It could be an old enemy who takes advantage of Percy's weakened state. For example, Kronos might see this as an opportunity to finally defeat Percy and take over Olympus. Since Percy has lost an arm, Kronos might think he's more vulnerable and launch a new attack.

It could be Orochimaru. He's always been interested in Naruto's power. Maybe he devises a plan to make Naruto lose his memory so he can manipulate him more easily, perhaps to use him as a weapon against the Hidden Leaf.

It could be Kaguya. If Kaguya were to return and use her powerful abilities to drive Naruto mad, perhaps by invading his mind with dark thoughts and memories. She has the power to warp reality and minds, so she could easily make Naruto lose his sanity. And since she has a grudge against the world of ninjas, making Naruto, one of the most powerful ninjas, go insane would be a great way for her to start her reign of terror all over again.

A new and mysterious dark ninja could be the main antagonist. This character might have a special ability to corrupt others. They target Naruto specifically because of his power and influence. By using some form of dark jutsu or psychological manipulation, they make Naruto lose his humanity, turning him into a weapon against the very people he used to protect.

It could be a new villain who has the power to manipulate memories. This villain might target Deku specifically because he sees Deku as a threat to his plans. He wants to erase Deku's memory so that he can't interfere with his evil schemes in the hero world.

The Akatsuki could be the main antagonists. They might think that Naruto's voice holds some key to a powerful weapon or secret. So they send one of their members, like Zetsu, to steal Naruto's voice. Zetsu's ability to blend in would make it easier for him to get close to Naruto and carry out the deed.