The Death Eaters are an obvious choice. They've always been after Harry, so they could easily be the ones to kidnap him. They'd want to take him to Voldemort to prove their loyalty and also to finally get rid of Harry for good.
Well, perhaps Voldemort's followers could kidnap Harry to use him as a bargaining chip. They might hold him in an old, dark wizarding house, guarded by dark magic. Harry would then try to escape using his own magic skills and the help of some unexpected allies like house - elves who secretly oppose Voldemort.
In such a fanfiction, Harry might be used as a bait to lure his friends into a trap. Voldemort could try to extract information from Harry about the Order of the Phoenix or Dumbledore's plans. Maybe he would also attempt to turn Harry to the dark side by showing him the so - called 'power' of the Dark Arts.
In many fanfictions where Gil gets kidnapped, it could be for various reasons. Maybe it's by a rival group who wants to use him as a bargaining chip against his friends. For example, in some stories, Uma's crew might be targeted because they are seen as a threat in the Isle of the Lost. Gil, being a part of that crew, gets kidnapped. Writers often use this plot device to create drama and show the loyalty and courage of the other characters as they try to rescue him.
One idea could be that Sam is kidnapped by a group of rogue hunters who think he's gone too far in his use of demon powers. They lock him in an abandoned bunker and try to 'exorcise' the 'demon' out of him, not realizing he's in control. His brother Dean has to race against time to find him, following a series of cryptic clues left by the kidnappers.
Natsu was walking through the forest one day when suddenly a group of dark mages emerged from the shadows. They quickly overpowered him with some sort of magic -binding spell. They dragged him away to an old, creepy lair. His friends back at the guild soon realized he was missing and set out on a search mission. Happy flew around frantically looking for any sign of his friend. Lucy, Gray and Erza teamed up and followed some leads they found in the forest. They were determined to rescue Natsu no matter what it took.
Peter's kidnapping was a carefully planned event. The kidnappers had been observing him for weeks. One day, as he entered a park, they grabbed him. They blindfolded him and took him to a secluded location. Peter was scared but he didn't show it. He started to befriend one of the kidnappers, a young guy who seemed a bit hesitant about the whole thing. Peter told him stories about his life and gradually the kidnapper started to have second thoughts. Eventually, the kidnapper helped Peter escape by distracting the others.
It could be a new character created specifically for the fanfiction. Maybe a young scientist who is working on a project related to the super - soldier serum. Since Bucky has a history with the serum, this would create an interesting dynamic.
In this fanfiction, the blurry vision glasses could be a family heirloom of Harry's that holds a great secret. A group of evil wizards kidnaps him to force him to reveal the secret of the glasses. Harry, being the brave character he is, tries to use his wits and the little magic he can muster with his blurry vision to break free and keep the secret safe.