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.
Sam is also a probable candidate. Given her feisty nature, it could lead to some interesting scenarios where she tries to fight back against the kidnapper. And her relationship with Carly and Freddie would drive them to take action.
One possible antagonist could be the Akatsuki members. They are always looking for strong opponents or valuable information. Maybe they target Iruka because they think he has some knowledge about Naruto that they can use, and Kakashi has to stop them.
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.
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 Naraku would be a likely kidnapper. He is always plotting against Inuyasha and trying to get his hands on the Tessaiga. Kidnapping Inuyasha would be a way for him to either steal the sword or find a way to break Inuyasha's spirit so that he can no longer oppose him.
There could also be a group of villains working together to kidnap Sonic. For instance, a combination of Eggman and some of his more powerful robot creations along with a couple of other villains from Sonic's rogues gallery. They might have a grand plan that requires Sonic's capture, like using his power to open a portal to a world full of chaos energy that they can then control.
A jealous rival of Inuyasha might kidnap Kagome. For example, another demon who wants Inuyasha for himself might think that by taking Kagome, he can get Inuyasha to do his bidding. This would create a lot of drama and conflict between Inuyasha and the kidnapper.
In many Dramione kidnapped fanfictions, Death Eaters or their remnants are common antagonists. They might want to use Hermione as a bargaining chip against the Order of the Phoenix or to gain some dark magic knowledge from her.
Typical antagonists in this kind of fanfiction can be various. The Sith, as we know, are always a likely candidate. They seek power and might see a kidnapped Jedi as a means to an end, like getting access to ancient Jedi secrets. But it's not just them. There could be rogue elements within the galaxy who oppose the Jedi for different reasons. Some might be power - hungry warlords who think a kidnapped Jedi can give them an edge in their territorial disputes. Also, there are the shadowy criminal organizations that might kidnap a Jedi for ransom or to sell on the black market. They don't care about the Jedi - Sith conflict; they just see an opportunity for profit or power.
Often, it's the hunters who turn out to be the antagonists. Hunters who don't believe in the good of the supernatural beings associated with Stiles. They might think that by kidnapping Stiles, they can get information about the other supernaturals and eliminate them. Also, some fanfictions feature a deranged human scientist who wants to study Stiles because he seems to be a key to understanding the supernatural world, but they go about it in a very unethical way by kidnapping him.