It might even be a human. In some fanfictions, a deranged human who has discovered the existence of vampires and werewolves could kidnap Bella in hopes of using her as a way to gain power or knowledge. This human could be a scientist or someone with a lot of resources who has been secretly studying the supernatural world and sees Bella as the key to unlocking more secrets.
Another possibility is a werewolf who has lost control or has a different agenda. In the Twilight universe, the werewolves and vampires have a tenuous relationship, and a werewolf kidnapping Bella could add a whole new layer of conflict. It could be a werewolf who doesn't trust Bella's relationship with the vampires and thinks she's a threat to their pack in some way.
It could be a rogue vampire who wants Bella for her unique blood. There are many vampires in the Twilight world who might be tempted by her smell. Another potential kidnapper could be a vengeful werewolf who blames Bella for something, perhaps for the way the vampires and werewolves' relationship has changed. It might also be a human who has somehow found out about the vampire world and thinks they can use Bella to gain power or knowledge.
One possible kidnapper could be a rival engineer who is jealous of MacGyver's fame and ability to solve problems so effortlessly. This person might think that by kidnapping MacGyver, he can steal his ideas and take credit for his inventions. Also, it could be a group of eco - terrorists who believe MacGyver's work has been harmful to the environment in some way and want to force him to make amends by using his skills for their cause. A third option is a deranged fan who has an unhealthy obsession with MacGyver and wants to keep him all to himself, thinking that kidnapping is the only way to have MacGyver's undivided attention.
Perhaps a dark guild that has been observing Fairy Tail for a long time. They see Mirajane as a valuable asset and believe that her powers can help them in their evil plans. So they plot to kidnap her and force her to cooperate.
It could be a group of rogue shinobi from a neighboring village who are jealous of Konoha's power and see Hinata as a valuable hostage. They might think that by kidnapping her, they can gain some leverage over Konoha in terms of resources or territory.
A powerful and vengeful warlord from a forgotten realm could be a kidnapper. He might see Wonder Woman as a threat to his plans of conquest and decides to take her out of the picture by kidnapping her.
It could be a rival vampire clan. They might see the son as a threat because of his unique nature, being part human and part vampire. They may think that as he grows up, he could become very powerful and pose a danger to their own power and territory in the vampire world.
One likely kidnapper could be Discord. He loves chaos and might see kidnapping Twilight as a great way to create chaos in Equestria. He could be bored and think that Twilight's absence would disrupt the harmony that she and her friends usually maintain. Another possible kidnapper is a jealous rival magician. Someone who is envious of Twilight's power and knowledge of magic and believes that by kidnapping her, they can steal her secrets and become the most powerful magic user in Equestria.
It could be a new vampire who is jealous of Edward's relationship with Bella. This vampire might be from a far - off place and has heard about the Cullens and Bella. They decide to kidnap Bella to make Edward suffer. Another possible antagonist could be a human who has a grudge against the Cullens. Since Bella is associated with them, they target her. For example, a human whose family was attacked by a vampire in the past and blames the Cullens for not preventing it. They think that by kidnapping Bella, they can get back at the Cullens.
It could be a rival ninja clan if this is in a ninja - themed story. They might see Sakura as a valuable asset to get information about her own clan or to use her skills against her people.
A new, evil sorcerer who has just arrived in Narnia could be the kidnapper. He might have heard tales of Lucy's importance and figure that by taking her hostage, he can control the forces of good in Narnia. Or perhaps a tribe of wild centaurs who have been corrupted by some dark magic might kidnap Lucy. They could be under the influence of a spell that makes them believe Lucy is the cause of all their problems and that getting rid of her will solve everything.