Perhaps there could be a new character introduced who becomes the hero. This character could be someone who has been observing Aoshi's actions from the shadows and decides to step in to save Kaoru when they realize the situation is getting out of hand.
Well, perhaps Aoshi kidnaps Kaoru because he believes she has some information crucial to his clan's honor. He might be acting out of a sense of duty that has been warped by his past traumas.
Typically, Mario would be the hero in a 'bowser kidnaps peach fanfiction'. He has a long - standing relationship with Peach and is always ready to rescue her from Bowser.
Perhaps a new character could be introduced as the hero. A wizard or a powerful human with some extraordinary skills. This new character could be someone who has been observing Peter's actions from afar and decides to step in to save Bella when the time is right.
It could be a rookie cop who's always looked up to Burgess. This rookie might have noticed something off about Gaines before the kidnapping and starts to dig deeper on their own. They could end up finding a crucial piece of information that leads to Burgess' rescue.
It could be another dark guild. They might be using Gray as a pawn to kidnap Lucy, perhaps because they know of her celestial spirit magic and want to use it for their own evil purposes. Gray may not be fully aware of their manipulation.
It could be Aizawa himself if he's being controlled by something or has some sort of ulterior motive that is not in Izuku's best interest. But if not Aizawa, then there could be a hidden villain who is manipulating Aizawa to kidnap Izuku.
It could be a new, powerful dark wizard who has a grudge against Fairy Tail. This wizard might manipulate Natsu into kidnapping Lucy to create chaos within the guild. The dark wizard could be after some powerful magic that only Lucy can access, and by kidnapping her, they hope to force her to give it up.
Well, Vegeta himself could be seen as the antagonist in the beginning. Since he is the one doing the kidnapping, he's causing distress to Bulma. However, as the story progresses, the real antagonist could turn out to be someone else. For example, another alien race that is after Vegeta and uses Bulma as bait to lure him into a trap. So, it's not so straightforward in a fanfiction.
The OC could be a mysterious ninja from a far - off land. Sasuke might think this ninja has some secret knowledge about the Uchiha clan that he wants, so he kidnaps them.
The rogue could be a disgruntled ex - knight who was wrongfully banished from his order. He sees kidnapping Lucy as a way to get revenge on the kingdom or the people who wronged him.