One possible fanfiction could be that Harry Potter is kidnapped by a dark wizard who has some strange power that makes Harry squint. Maybe this wizard is trying to use Harry's unique connection to Voldemort for some dark ritual. Harry has to use his wits and the few spells he can manage while squinting to try and escape.
The Death Eaters are very likely kidnappers. Since they are Voldemort's followers, they might try to kidnap Harry to please their master or gain some advantage in the wizarding war. Another possibility is some power - hungry pure - blood wizards who see Harry as a threat to their status quo. They could kidnap him to keep him out of the way.
At the beginning, Harry would be frightened. He's in an unknown place with Loki, who is known for his trickery. But Harry is no pushover. He would quickly gather his courage. He would remember all the times he's faced danger at Hogwarts. He might try to communicate with Loki to understand his motives. If Loki reveals that he wants to use Harry's magic for something, Harry would try to find a way to outsmart Loki. He could pretend to cooperate while actually looking for a weakness in Loki's plan. Harry would also be thinking about his friends at Hogwarts and how he needs to get back to them, which would give him extra motivation to fight back against Loki.
Well, there's a fanfiction where Harry is kidnapped by a group of rogue Aurors who have turned bad. They believe Harry holds the key to some ancient magic they want to control. There are also stories where Harry is kidnapped by magical creatures who mistake him for someone else. For example, a group of centaurs might think he's a threat to their forest and take him captive to figure out his intentions. In these fanfictions, Harry usually has to use his wits and magic to escape.
Well, it's fairly common. Fanfic writers often use this as a means to create drama and tension. Since Sirius is such an important and mysterious figure in Harry's life, the idea of him kidnapping Harry can lead to all sorts of interesting storylines. It can also be a way to rewrite parts of the original story, perhaps to give Sirius more agency or to create a different kind of adventure for Harry.
I'm not entirely sure as 'fanfic' can vary greatly. It could be about Harry Potter being kidnapped in some alternate storyline created by a fan. Maybe it's a dark plot where Voldemort or some new villain has abducted Harry for their own evil purposes.
Well, if Harry Potter was kidnapped by Loki, it could be a total chaos. Loki might use Harry's magic for his own devious plans. Maybe he wants to take over Hogwarts using Harry's knowledge of the school's secrets. Or he could be trying to trade Harry for something valuable in the wizarding world. Another possibility is that Harry would try to outwit Loki with his own brand of cleverness and magic, leading to a battle of wits between the two.
Perhaps in the fanfiction, Harry is kidnapped by a powerful magical creature that has a vendetta against wizards. When it grabs Harry, his glasses are knocked off. The creature takes him to its underground den. Harry, without his glasses, is at a great disadvantage as he can't see well. But he remembers the magic within him. He tries to communicate with the creature using Parseltongue (if it's a snake - like creature). He hopes to either convince it to let him go or at least find his glasses so he can better defend himself and find a way back to Hogwarts.
In this kind of fanfiction, it could be that Dumbledore has some very complex motives for 'kidnapping' Harry. Maybe he wants to protect Harry from a danger that he can't explain openly. For example, there might be a powerful dark wizard who has found a way to spy on Hogwarts and Dumbledore believes that keeping Harry in a secret location is the only way to keep him safe.
Another aspect could be that Dumbledore is being controlled or influenced by some external force. In the fanfiction, this could lead to a very interesting story where Harry has to figure out how to break Dumbledore free from this control while also escaping from his 'kidnapper'. It would be a total role - reversal from the original story.