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 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 rogue vampire. Maybe a vampire who has a grudge against the Cullens and figures that taking Bella is the best way to get back at them. This vampire might be acting alone, thinking that he or she can use Bella to bargain with the Cullens for something they have.
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.
Perhaps an escaped convict who has a grudge against the Reagan family in general and sees Jamie as an easy target. This convict might be looking to use Jamie as a bargaining chip to get out of the country or something similar.
It could be a rival movie studio. They see Tom Holland as a threat to their own leading actors. So, they arrange for his kidnapping to disrupt his career and the production of his movies. Also, it could be a criminal gang that randomly targets celebrities for ransom. Tom Holland just happens to be their next victim. They don't care about his fame, just the money they can get from his kidnapping.
A rogue shinobi who has a personal vendetta against Sakura or her friends. Maybe this shinobi blames Sakura for something that happened in the past and decides to kidnap her as revenge. He could be a powerful and unpredictable antagonist.
It could be Doji. He's always been after powerful beyblades and Gingka's Pegasus is one of the strongest. He might kidnap Gingka to get Pegasus and use its power for his own dark plans. Another option is Ryuga. Although he has his own code, in some fanfictions he could be desperate enough to kidnap Gingka to prove his dominance in the beyblade world. Also, a new and unknown blader who is jealous of Gingka's skills and popularity could be the antagonist and kidnap him to make a name for himself.
The main antagonist could be Shere Khan. He has always been a threat to Mowgli and might see kidnapping him as a way to finally get rid of the 'man - cub'. He could be working with some of his sly jackal minions to carry out the kidnapping.
It could be Discord. He loves causing chaos and kidnapping Twilight Sparkle would be a great way to disrupt the harmony in Equestria. He might do it just for fun or to see how Twilight's friends would react.
It could be a powerful vampire who doesn't like Bella's new - found strength. This vampire might see her as a threat to their own power and control in the vampire world.