Potential kidnappers could include a vampire from a different coven who is envious of the Cullens' relationship with Bella. They might see her as a way to gain an advantage over the Cullens. Also, it could be a new supernatural creature that we haven't seen much in the original Twilight series. This new creature could have its own motives for taking Bella, like using her as a bargaining chip or as an experiment. There's also the possibility of a human with a grudge against Bella or the Cullens who teams up with a supernatural being to carry out the kidnapping.
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.
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.
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.
The potential kidnappers could be a secret society within the wizarding world that has its own agenda. They might believe that Hermione has a special power or connection that could either help or harm their cause, so they took her at birth to control that. It could also be a non - human magical being, like a centaur or a goblin, who thought that Hermione's birth was a sign of something significant in the magical world. They kidnapped her to study her or use her in their own cultural or magical practices. Or perhaps it was a wizard who had a grudge against Hermione's parents and took their child as revenge.
It could be a group of rogue demigods who are jealous of Percy's status and power. They might think that by kidnapping and torturing him, they can gain some of his abilities or just to prove that they are stronger.
It could be a group of rogue Death Eaters who want to use baby Harry as a bargaining chip against Voldemort. They might think that by having Harry, they can gain power within the dark wizard's ranks or even overthrow him.
The Sith could be potential kidnappers. They might see Princess Leia as a powerful Force - user and want to either turn her to the dark side or use her in some dark ritual. Another possibility is the bounty hunters. They could be hired by some powerful and shadowy figure to kidnap Leia for various reasons, like for political leverage or to get information from her. Also, a new and unknown alien race that has just entered the galactic scene could be the kidnappers. They might not understand the significance of Leia but kidnap her out of curiosity or because they misinterpret her actions as a threat.
A new, unknown criminal organization could be behind the kidnapping. They might have heard about Duke's potential and see him as a valuable asset. They could be planning to either brainwash Duke to join them or use his abilities to carry out their illegal activities.
The Akatsuki could be potential kidnappers. They are always on the lookout for powerful jinchuriki like Naruto to extract the tailed beasts from them. Orochimaru could also be a kidnapper as he is always interested in obtaining powerful bodies and Naruto's strength would be very appealing to him.
It could be a dark organization that has been secretly operating in the shadows. They kidnap Naruto because they believe he has some sort of connection to an ancient prophecy that they want to either fulfill or prevent, depending on their motives.
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.