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.
Perhaps an ancient and powerful snake that lives deep in the jungle could be the main antagonist. It sees Mowgli as a disruption to the natural order of the jungle and decides to have him kidnapped. The snake could use its hypnotic powers to command other animals to carry out the deed.
A human hunter could be the main antagonist. He might be a cruel and greedy man who has heard of Mowgli and wants to capture him to show off in the civilized world or sell him to a circus. He could hire some local tribesmen who are not as friendly towards Mowgli's jungle family to do the actual kidnapping.
It could be a rogue ninja from Kiri who has his own motives for kidnapping Naruto. Maybe he wants to use Naruto's power to take over Kiri. He sees Naruto as a tool to gain control and doesn't care about the consequences for the relations between Konoha and Kiri.
It could be an old enemy of the gods who has been biding their time. Maybe a powerful Titan that was thought to be defeated but has found a way to come back. This Titan could use Percy to kidnap Reyna because she has a connection to a certain power that the Titan wants to use to regain full strength and overthrow the gods once again.
Obviously, Red X would be the main antagonist as he is the one doing the kidnapping. But there could also be other antagonists in the background. For example, if Red X was working for someone else, that person could also be an antagonist. Maybe a powerful intergalactic criminal who wants Starfire's powers for his evil plans.
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.
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.
Kakuzu might also be a main antagonist in this fanfiction. He's always after money and power. Maybe he sees Sakura as a valuable asset, either to sell her back to Konoha for a large sum or to use her skills for his own gain. He's not very empathetic and would likely be ruthless in his treatment of her.
It could be Voldemort or one of his Death Eaters. They might be using Harry as a pawn to get to Parvati for some reason, like she has a special power or knowledge that they want.