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.
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 plotline could be that Harry is under some sort of dark magic influence or curse that forces him to aid in Voldemort's resurrection. Maybe a powerful dark wizard has manipulated Harry's mind.
Yes, there are some out there. Some fanfictions might explore the idea of Harry Potter helping Voldemort in an alternate universe where their goals are misaligned or there are complex magical reasons for it. For example, in a situation where Harry believes he can change Voldemort's ways by getting closer to him, rather than fighting him directly.
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.
Themes could include the blurred lines between good and evil. If Harry is a fan of Voldemort, it challenges the traditional view of their roles. Another theme might be the exploration of dark magic. Since Voldemort is associated with it, Harry's fascination could lead to a deeper look into the nature of dark magic.
The wizarding world might be in shock. People who have always seen Voldemort as the ultimate evil would be confused. They would have to re - evaluate their views on Voldemort. Harry, on the other hand, would be in a very strange situation. He might be distrusted by some of his friends who think he has made a deal with Voldemort.
One common theme is redemption. The idea that Voldemort can be saved or changed through Harry's help. Another theme is the exploration of power dynamics. Since Harry and Voldemort have very different views on power, seeing how they interact when Harry is trying to help can be really interesting.
One possible plotline could be Harry being held captive by Voldemort in a way that's like a pet. Voldemort might use Harry to show his power over the Boy Who Lived. Another could be that Harry has some sort of magic that makes Voldemort keep him as a pet instead of killing him immediately.
In some stories, their friendship could start from a time - travel accident. Harry gets sent back in time and ends up befriending a young Voldemort. He tries to influence Voldemort's path, and they form an unlikely bond. Harry might introduce Voldemort to different values and ideas that were not present in Voldemort's original upbringing, which causes a major shift in the future of the wizarding world.
One interesting plot could be that they are distant relatives through some ancient wizarding bloodline. Maybe there was a curse placed on the family long ago that led to their enmity.