Perhaps an old enemy of Deku who has a grudge against him. This antagonist might have found a way to use a powerful artifact or quirk to make Deku lose his memory. Their goal could be to break Deku's spirit and make him give up on being a hero. They might also try to turn Deku's friends against him while he's in his vulnerable state.
There could be a group of villains working together. One of them has the ability to erase memories, and the others see the opportunity to use Deku's memory loss to their advantage. They might try to make Deku believe that he was one of them all along, and persuade him to use his powers for evil. This would create a really interesting conflict as Deku has to fight against false memories and the influence of these villains.
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 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 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.
Well, one possible plot could be that Deku gets hit by a villain's quirk during a battle. This quirk erases his memory. His friends, like Uraraka and Iida, then try to help him remember who he is. They might show him his hero notebook, hoping it jogs his memory about his dream of becoming a great hero.
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 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 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.
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.
Voldemort could be the main antagonist. He might have seen the potential in Harry's twin and decided to get rid of the twin to weaken Harry or use the twin for his own dark purposes.