It could be Frieza. Frieza has a long - standing grudge against the Saiyans, especially Vegeta. He might kidnap Vegeta to torture him or to use him as a bargaining chip against the other Saiyans. Another possibility is Cell. Cell might see value in Vegeta's genetic material and kidnap him to extract it for his own evolution.
The kidnapper could be an evil wizard or sorcerer. In some fanfiction universes, magic and superpowers co - exist. This wizard might be after Vegeta's life - force to fuel his dark magic spells. He could have lured Vegeta into a trap and kidnapped him. Or it could be an android created by a mad scientist who wants to study Vegeta's combat abilities and use them to upgrade other androids.
The kidnapper might be a deranged Toad. Maybe this Toad has been corrupted by some evil force and decides to kidnap Luigi for reasons only known to the twisted mind of this new 'evil Toad'.
It could be Dr. Eggman. He's always after Sonic and would love to get his hands on him to stop Sonic from foiling his evil plans. He might want to study Sonic's speed and use it for his own machines.
The kidnapper could be a former ally who has turned against Luffy. Maybe they've been corrupted by power or some other force. They believe that by kidnapping Luffy, they can take over the Straw Hat Pirates or get their hands on valuable resources that Luffy has access to.
It could be a rival guild member who wants to use Juvia as leverage against Fairy Tail. Maybe someone jealous of her powers or her relationship with Gray.
Maybe it's a group of anti - Sailor Scouts. They are people who oppose the Sailor Scouts' mission of protecting the world and think that by kidnapping Sailor Moon, they can stop the Scouts' activities. They might be under the influence of an ancient evil.
There are several candidates. It could be a rogue demigod who has turned to the dark side. They might think that by kidnapping and torturing Annabeth, they can gain power or recognition. It could also be a minion of a major god who is acting on their master's orders, perhaps because Annabeth has somehow angered that god. Additionally, it could be a mysterious figure from a long - forgotten prophecy who believes that Annabeth is the key to fulfilling their dark desires.
It could be another werewolf from a different pack. There are often rivalries between packs in the Teen Wolf universe, and someone might see Liam as a way to gain an advantage over his pack.
In some 'damian gets kidnapped fanfiction', the kidnapper could be an otherworldly being. Let's say an alien or a magical creature that has an interest in Damian for some strange reason. It could be that Damian has a unique energy or power that the otherworldly being wants to study or harness. Or perhaps it's a human who has been corrupted by some dark magic or technology and is acting on behalf of a more powerful, hidden force that wants Damian for its own nefarious purposes.
Maybe it's a group of poachers who are after rare Pokémon. Gengar, being a relatively unique and powerful ghost - type, would be a valuable catch for them. They could plan to sell Gengar on the black market for a high price.
It could be a jealous rival of Felicity. Maybe someone who is envious of Felicity's intelligence which is somehow related to the glasses. So they kidnap the glasses to try and bring Felicity down.