One possible plot could be that Harry is captured by Voldemort and his friends have to mount a dangerous rescue mission. They might have to infiltrate Voldemort's stronghold, which could be filled with dark magic defenses.
Perhaps in the fanfic, Voldemort has found a way to nullify Harry's magic for a short time. Without his magic, Harry is just a normal boy and Voldemort takes this opportunity to beat him to death. It could also be that Voldemort has an unexpected accomplice who helps him corner Harry, leading to Harry's demise.
In the 'Harry gets captured by Voldemort fanfiction', Harry could find an unexpected ally within Voldemort's ranks. Maybe a Death Eater who has a change of heart. This ally could help Harry escape or at least send messages to his friends. Another aspect could be that Voldemort performs some dark magic on Harry that gives him strange powers or visions. Harry then has to figure out how to use or overcome these new abilities while still being held captive. It could also be that the wizarding world outside is in chaos due to Harry's capture and different factions start to emerge, some wanting to negotiate with Voldemort for Harry's release and others wanting to go to war immediately.
One possible plot could be that Harry is magically compelled to submit. Voldemort might use some dark magic or a powerful artifact to control Harry's mind. Another plot could be that Harry submits out of a false belief that it will save his friends. For example, Voldemort could deceive Harry into thinking that if he surrenders, his friends will be spared. Or perhaps Harry is so worn down by the fight that he gives in, thinking there's no other way, and then the story could explore how Voldemort uses Harry's submission to his advantage, like making Harry a tool to gain the loyalty of other wizards.
In such a fanfic, Harry might be tortured for information about the Order of the Phoenix. Voldemort could try to turn Harry to the dark side, using his powerful magic to break Harry's will.
One of the main themes is often the power of friendship. Even when captured, Harry's friends would be fighting for him. Also, the theme of good versus evil is prominent. Harry represents good, and Voldemort is the ultimate evil. The fanfic might show how Harry resists the dark influence of Voldemort. Additionally, themes like hope and perseverance are likely to be there. Harry doesn't give up easily, and that gives hope to the wizarding world that he can still overcome Voldemort, even when in captivity.
One possible plot could be Voldemort brainwashing Harry to turn against his friends at Hogwarts. Harry might start acting strangely, like isolating himself from Ron and Hermione. Another plot could involve Voldemort using the brainwashed Harry to steal important magical artifacts from the school. Maybe Harry is made to believe that these artifacts are rightfully Voldemort's and that he's on a noble mission.
One interesting fanfiction could be 'The Dark Captivity'. In it, Harry is held in a magically fortified dungeon by Voldemort. Voldemort's motives for capturing Harry are explored in depth, not just about killing him but also about breaking his spirit. Another could be 'Captive Potter'. Here, Harry endures various mental tortures while being Voldemort's captive, but he manages to secretly communicate with his friends using a very unique and magical way.
Ron would also be important. His loyalty to Harry is unwavering. He might be the one who bravely volunteers to go on the most dangerous part of the rescue mission, perhaps facing off against some of Voldemort's most feared Death Eaters.
One possible plot could be Harry trying to understand Voldemort's past and motives through the letters. Maybe he writes to ask about Voldemort's childhood, what led him to become so evil. Another could be Harry attempting to persuade Voldemort to change his ways, using the letters as a form of communication to reach the dark wizard's humanity. Or perhaps Harry writes to taunt Voldemort, using the fanfiction as a way to get under his skin and make him angry in a psychological battle.
One possible plot is that Harry discovers a hidden prophecy that says only by helping Voldemort can he truly save the wizarding world in the long run. So, he reluctantly starts to assist Voldemort in his conquests, all while secretly planning to turn the tables.