It could be an old enemy of the Power Rangers who has a grudge against them. Maybe a villain they thought they had defeated before.
In this fanfiction, the kidnapper might be a new, mysterious character. This character could be from another dimension and has come to the bridge to kidnap someone as part of a grand plan to gain power over the Power Rangers' world. They might have special powers or abilities that the Rangers have never seen before, which would make the rescue mission even more difficult. Another possibility is that it could be a group of smaller villains who have banded together specifically to target the bridge and the person there, thinking that by kidnapping this person they can force the Power Rangers to do their bidding.
Maybe in this fanfiction, the bridge is a special, magical bridge. The kidnappers chose it because it has some sort of power they want to use for evil. The Power Rangers would first need to figure out why the bridge was targeted. They could call on their Zords and use their advanced technology to scan the bridge for any hidden secrets. Along the way, they might encounter some allies or old enemies who could either help or hinder their mission to rescue the kidnapped person.
It could be an old enemy of Keith's, someone from his past who has a vendetta against him. Maybe a former rival from his days in the military academy who has now turned to more devious means.
In some creative fanfiction, it could be an alternate version of Spiderman himself from a different dimension. This Spiderman has gone bad and kidnaps a fan from our dimension as part of some grand, dark plan that involves crossing dimensions and causing chaos.
A power - hungry nobleman from a neighboring land could be the kidnapper. He might believe that by having Rapunzel in his possession, he can gain some sort of political advantage or use her as a bargaining chip against her kingdom.
It could be Orochimaru. He's always been interested in obtaining powerful individuals for his experiments, and Naruko with her unique abilities would be a prime target.
It could be a new villain that has emerged, someone who is jealous of Gohan's power and wants to make it his own. This villain might be from a far - off planet and has heard of the Saiyan legend.
It could be a group of rogue Decepticons who are acting on their own. They might not follow Megatron's orders but still see value in kidnapping Arcee. Maybe they have a personal vendetta against her or think they can get rich by selling her to the highest bidder in the Cybertronian underworld.
Maybe it's a group of villains working together. They could be from different factions but united by the idea that Izuku has the potential to become a huge threat to their plans. They might kidnap him to either eliminate him or turn him to their side. Also, it could be a lone, obsessed fan of a villain who wants to prove themselves by bringing Izuku to their idol.
A group of outcasts from different islands might kidnap Moana. They feel that they have been wronged by the 'normal' society that Moana represents. They hope that by kidnapping her, they can get the attention they deserve and force the islands to change their ways. But Moana listens to their grievances and tries to find a way to help them while also finding a way home.
It could be Zeref. Since Zeref has an interest in powerful magic users and Natsu is a strong dragon slayer, Zeref might kidnap Natsu to study his magic or use him in some dark plan related to his quest for immortality or power.