Writers often start with a unique concept. For example, they might think about how Marie Potter could interact with Voldemort in a way that defies the original canon. They could then build a plot around that, like Marie Potter having a special ability that Voldemort wants to use for his own ends.
Some popular ones might be found on fanfiction platforms. You can search for tags like 'Marie Potter' and 'Voldemort' on websites such as Archive of Our Own. However, the availability and popularity can vary greatly depending on how the authors have developed their unique takes on this rather unusual pairing.
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.
One interesting plot could be Lily's struggle with her identity. Being the daughter of Voldemort, she might constantly battle between the dark side that is in her blood and the good she has been taught. For example, she could be tempted by dark magic but resist it because of the friends she has made at Hogwarts.
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.
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.
Some writers might show it through a strange attraction. Despite their hatred, there could be an underlying pull towards each other. For example, they might find themselves in situations where they can't bring themselves to kill the other when they have the chance.
Writers can start by creating a unique scenario for Law and Nami to meet. For example, instead of the normal One Piece encounters, they could meet in a parallel world. Then, introduce a conflict like a powerful villain who targets Nami because of her skills and Law has to protect her. Also, adding some twists to their relationship, like Nami initially distrusting Law but then slowly realizing his good intentions.
Writers can start by creating a unique setting. For example, a new planet or a hidden dimension where Ben and Ester find themselves. Then, introduce a conflict. It could be a powerful villain who has a special connection to either Ben or Ester. Another way is to play with their personalities. Maybe Ester is really scared at first but has to overcome her fear to help Ben. Writers can also add some twists and turns, like a character who seems to be a friend but turns out to be an enemy in the middle of the story.
Writers often start with the basic concept of bringing Naruto and Tia Harribel together. They might think about how these two characters would interact based on their personalities. For instance, Naruto's cheerful and determined nature against Harribel's more reserved and powerful demeanor. Then, they build a plot around a central conflict. It could be an external threat that forces them to team up.