In Kidnaped Izuku Fanfiction, likely antagonists could be rival villains who see Izuku as a threat to their plans. For example, those who want to stop him from becoming a great hero. It could also be villains who are after his unique quirk. Maybe they want to study it or use it for their own evil purposes. Another possibility is that it could be a group with a vendetta against the hero school he attends, and they target Izuku to send a message.
One common theme is the test of Izuku's heroism in captivity. Even when kidnapped, he might try to save other captives or find a way to stop the villains. Another theme could be the exploration of his relationships. For example, how his friends and mentors react to his kidnapping and how they strive to rescue him. Also, it could be about Izuku's growth in a difficult situation. He may learn new skills or gain a deeper understanding of his powers while being held against his will.
If Izuku is an alien, the antagonists could be other aliens who want to use his unique powers for their own evil purposes. Maybe they see Izuku as a key to conquering Earth or other planets.
The Akatsuki could also be antagonists in an Iruka kidnapped fanfiction. They might believe that Iruka has some information about the jinchuriki, especially since he is close to Naruto. Or there could be a new, made - up antagonist in the fanfiction, like a powerful dark wizard who has set his sights on Iruka for some unknown but sinister reason. This new antagonist could add a fresh and unique element to the story.
It could be the traditional villains from the original story, like the Titans or some powerful monsters. They might torture Percy to get information or just to cause chaos.
All Might would probably be in it. Since he's Izuku's mentor, he could play a role in helping Izuku understand and control his time - controlling power. Maybe he has some past experience related to time powers that he shares with Izuku.
Often, in 'izuku super soldier fanfiction', there are villains from the original series that become more menacing when facing Izuku as a super soldier. For example, All For One might be an even greater threat as he tries to either control or destroy Izuku.
Draco Malfoy is often an antagonist in such fanfictions. His family's beliefs in pure - blood supremacy could lead him to target Hermione, who is a Muggle - born witch. He might use his influence and his Slytherin friends to make her life difficult.
Perhaps some rogue ninja from other villages. They might see Naruto as a symbol of Konoha's power and want to take him down to weaken Konoha. These rogue ninjas could be led by a powerful and vengeful shinobi who has a grudge against Konoha. So when Naruto is alone on a mission or something, they attack, and that's when Minato steps in to save him.
It could be some rogue ninja clans that are after Karin for her special abilities. They might see her as a tool to gain power or information. For instance, a clan that specializes in blood - related jutsu might want to use Karin's unique blood - sensing ability for their own evil purposes.
It could be a group of dark wizards who have escaped from Azkaban and formed a new dark alliance. They might be seeking revenge on Harry and Hermione for their role in defeating Voldemort.