There's also the possibility of a secret organization. These types of groups often operate in the shadows. They could have some grand plan that involves Sarada, like using her to experiment on the Sharingan or something related to the power that runs in her family.
Maybe an enemy from another village. If there are political tensions between villages, kidnapping Sarada could be a way to gain an advantage. They might think that by having her, they can force her village to make concessions in treaties or resources.
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.
It could be the Joker. He's always up to no good and kidnapping Robin would be a great way to mess with Batman. The Joker might want to use Robin to force Batman into a series of cruel and twisted games.
A new villainous trainer group. They might see Ash as a threat to their plans of dominating the Pokémon world, so they kidnap him to stop him from competing in important battles or tournaments.
Maybe it's a group of rogue Death Eaters who are still loyal to Voldemort's ideals. They believe that by kidnapping Ginny, they can disrupt the peace in the wizarding world and send a message to those who defeated Voldemort.
One possibility is Dr. Eggman. He has always been after Sonic and wants to stop him. Kidnapping Sonic would be a way for him to achieve his evil goals.
Another possibility is that it's a group of mercenaries hired by a powerful underworld figure. The figure might want Nightwing out of the way so they can carry out their illegal activities more easily. The mercenaries would be well - trained and equipped, making it a difficult situation for Nightwing to escape from.