In a fanfiction, it could be a shadowy organization. They might have heard rumors about Raven's unique powers and decided to take her. This organization could be researching ways to gain superhuman abilities and believe Raven holds the key. Or it could be a deranged fan of Raven's powers. He might be obsessed with her and think that by kidnapping her, he can have her all to himself. Additionally, a vengeful spirit from Raven's past could be the kidnapper, seeking revenge for something she did long ago.
Enemies from her past, such as old villains she's fought against, would be likely kidnappers. They might hold a grudge for her previous defeats and want to use her for revenge or some dark purpose.
Orcs would be a likely kidnapper. They have a long - standing enmity with the elves, and Legolas being an important elf, would be a valuable captive. They could either want to torture him for information or use him as a bargaining chip with the elves.
It could be an obsessed fan. You know, someone who has taken their admiration too far. They might think that by kidnapping Liv and Maddie, they can have them all to themselves and be part of their exclusive world. This type of kidnapper would be really unpredictable as they are acting out of an extreme and abnormal form of love.
It could also be a secret organization within the Naruto universe that we don't know much about in the original story. This organization might be aware of some hidden prophecy or potential in Naruto related to the Nine - Tailed Fox. They kidnap him with the intention of either preventing that potential from being realized (if they see it as a threat) or using it for their own mysterious ends.
Perhaps it's an organization that wants to use Kakashi for their own evil plans. They might think that Kakashi's skills and knowledge would be useful for them. For example, they could be trying to create some sort of super - weapon and they need Kakashi's expertise in ninjutsu.
It could be Hades. He's always been an antagonist in the series and might kidnap Pit to mess with the gods. He could be thinking that by taking Pit, he can gain an advantage in his ongoing feud with them.
It could be a rogue Saiyan who wants to take over the Saiyan throne and thinks kidnapping Goku is the first step.
It could be a new villain that we haven't seen in the main Mario games. Maybe a powerful sorcerer who needs Mario for some dark magic ritual. This sorcerer might have been lurking in the shadows of the Mushroom Kingdom, waiting for the right moment to strike and kidnap Mario.
It could be a jealous rival of Esme. Maybe someone who is envious of her popularity or success.
Maybe it's a new villain created specifically for the fanfiction. This new character could be jealous of Sora's popularity and power, and decides to kidnap him to prove their own worth. They might think that by capturing Sora, they can gain the respect of other villains or even take over the worlds that Sora has been protecting.