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.
A new, unknown villain could be the main antagonist. This villain might have a vendetta against Naruto specifically because of his status as a hero in the Hidden Leaf. By making him lose his memory, the villain hopes to destroy Naruto's reputation and the hope he represents for the village, all while remaining in the shadows and causing chaos.
The Akatsuki could be the main antagonists. They might see Naruto's memory loss as an opportunity to turn him against his own village. They could try to fill his mind with false memories and convince him to join their cause, believing that without his real memories, he'd be more susceptible to their influence.
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.
It could be a marine admiral who has developed a new weapon that not only causes physical harm but also memory loss. This admiral might see Luffy as a big threat and decides to use this weapon on him to weaken the Straw Hat crew.
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.
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.
It could be a new, powerful ninja from a hidden village. This ninja has a special ability to manipulate the chakra related to emotions. He sees Naruto as a threat because of his popularity and power, so he uses his ability to make Naruto lose his emotions, hoping to weaken him and his influence.
It could be a rogue ninja from a hidden village that has a grudge against Naruto. This ninja might have a special jutsu that specifically targets Naruto's left eye during a fight, causing him to lose it.
It could be a new and powerful rogue ninja who manages to overpower Naruto and cause him to lose his arm. Maybe this antagonist has some sort of vendetta against Naruto or the Hidden Leaf village.
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 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.