One likely antagonist could be Chazz Princeton. He's often shown as a bit of a bully in the Yugioh series, and he might target Syrus out of jealousy or just to assert his dominance. Another could be an original character created by the fanfiction writer, perhaps a new student at Duel Academy who has a vendetta against Syrus for some reason. The Society of Light could also be antagonists in a sense, as they might try to manipulate Syrus or use him in their schemes, which could lead to him getting hurt.
One common plotline could be Syrus facing bullying at Duel Academy which causes him emotional hurt. Maybe some upperclassmen pick on him for his dueling skills or his timid nature. Another could be Syrus getting hurt in a dangerous duel, like against a powerful opponent who uses underhanded tactics. Also, he might get his heart broken in a romantic sub - plot within the Yugioh world, perhaps by someone he has a crush on who doesn't return his feelings.
Well, it could be Orochimaru. He was always up to no good and had an interest in powerful ninja. He might target Naruto and Temari to either steal their powers or use them for his evil experiments, which would surely lead to them getting hurt.
A plot could involve Hell Kaiser being sent to evaluate Syrus for some special training program. As he watches Syrus, he discovers that Syrus has some unique qualities that could be both an advantage and a disadvantage. There could be a conflict where Hell Kaiser has to decide whether to report accurately or help Syrus overcome his weaknesses first. The story could also include duels where Syrus shows his growth under Hell Kaiser's watchful eye.
One possible plot could be that Syrus, while exploring an ancient library in the Duel Academy, stumbles upon a hidden spellbook that has a connection to the Dark Magician Girl. He accidentally summons her in a weakened state. Then, they have to work together to regain her full power while facing various challenges from other duelists who want to capture the Dark Magician Girl for their own purposes.
Physical hurt might come from a powerful magic spell. Let's say an evil sorcerer casts a spell that drains Yami's energy and causes him pain. His struggle to overcome this and regain his strength could be a central part of the fanfiction.
Yes, there are likely some out there. You can start by checking fanfiction websites like FanFiction.net or Archive of Our Own. Just search for 'hell kaiser watches syrus' in the Yugioh section and you might find some interesting stories.
Their interaction could start with Syrus having a strange dream about the Dark Magician Girl. When he wakes up, he starts noticing little signs around the Duel Academy that seem to be related to her. One day, while dueling, he makes a move that somehow creates a small rift in the magic world, and the Dark Magician Girl steps through it for a brief moment. That moment of eye contact is the start of their interaction.
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.
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.
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.