It could be some dark wizards who are always looking for powerful magical items. They might have heard rumors about Harry's glasses having some special properties and decide to kidnap him to get their hands on the glasses.
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.
It could be about Harry having some sort of magical ailment that causes his vision to be blurry. Maybe it's a side - effect of a new spell he's been experimenting with or a curse from a dark wizard. Without reading the actual fanfiction, it's hard to say for sure.
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.
If this is a fanfiction, it might explore themes of vulnerability and heroism. Felicity being kidnapped puts her in a vulnerable position. Blurry without glasses could be seen as less capable at first, but perhaps they overcome this disadvantage to rescue Felicity. It could also involve a lot of character development for both Felicity and Blurry as they face this difficult situation.
Maybe it's a deranged fan of one of the Core Four. Somebody who has an unhealthy obsession and thinks that by kidnapping them, they can have a special relationship with their idol. It could be a fan of Archie's football career or Veronica's glamorous lifestyle. They might be living in their own deluded world and believe that the Core Four will come to love them if they keep them hostage.
It could be Discord. He loves chaos and might think it's fun to kidnap Twilight and see how her friends react. He could also be after her knowledge of magic to cause more mischief in Equestria.
We might expect a lot of mystery. Since it's about Harry having blurry vision, the author could be building up to a big reveal about what's causing it. It could be a new dark magic threat.
It could be some Death Eaters. They might want to use Harry as a bargaining chip against Dumbledore or the Order of the Phoenix. Without his glasses, Harry would be at a disadvantage against their dark magic. Another possibility is some rogue wizards who are after the fame and fortune that could come from kidnapping the famous Harry Potter.
It could be some rogue Death Eaters. They might think that kidnapping Harry without his glasses would make him more vulnerable and easier to control or deliver to Voldemort. They could be trying to prove their loyalty to the Dark Lord by bringing him Harry in a weakened state.
It could be Draco Malfoy. He's always been a bit of a troublemaker when it comes to Harry. Maybe he used a spell to knock the glasses off Harry's face during a confrontation in the hallway.