Perhaps Harry has been abused by a group of rogue wizards who are after his fame. Voldemort, seeing an opportunity to gain some sort of advantage, saves Harry. This starts a journey where Harry and Voldemort form an uneasy alliance. Voldemort may try to manipulate Harry's trust, while Harry tries to figure out Voldemort's true motives and also how to use this situation to make the wizarding world a better place.
One plot idea could be that Voldemort saves Harry from his abusers in the Dursley household because he realizes Harry's potential is being squandered there. He then takes Harry under his wing, not to turn him evil, but to train him in magic in a very different way than Hogwarts. Maybe he wants to use Harry as a tool against the Ministry, which he believes is corrupt.
Another possibility is that Harry is being physically and emotionally abused at Hogwarts by some dark forces that have infiltrated the school. Voldemort, for his own mysterious reasons, decides to rescue Harry. This could lead to a complex relationship where Harry is both grateful and conflicted, and Voldemort has to deal with his own Death Eaters who don't understand his new 'ally'.
Harry's character would likely become more complex. Initially, he might be distrustful of Voldemort's motives but also feel a sense of obligation for being saved. This could lead to him being more cautious and calculating in his actions, as he has to navigate the dangerous waters of being associated with the Dark Lord.
One possibility could be that Voldemort has a sudden change of heart. Maybe he realizes that Harry is the only one who can truly understand his loneliness and power. So instead of killing Harry, he decides to save him from a dangerous situation, like a powerful dark magic that even Voldemort fears could destroy the wizarding world if not controlled, and in the process, they form an unlikely alliance.
One possible plot point could be Voldemort's change of heart. Maybe seeing Harry being abused makes him question his long - held enmity towards Harry. He might start to see Harry as more of a victim than an enemy, which could lead to some really strange alliances or at least a change in his approach towards Harry.
In some fanfictions, Voldemort might save Harry due to a complex change in his character. For example, he could discover a newfound sense of respect for Harry's power or courage. Maybe he realizes that Harry is the only one who can truly understand him, and in a moment of strange kinship, he decides to save Harry from a greater threat that is about to befall them both.
Another plot could be that they are married in an alternate universe where their backstories are different. Maybe Voldemort was never as consumed by dark magic from the start. Harry and Voldemort could then discover that they have a deeper connection than they thought, and their marriage could be a journey of self - discovery for both of them, as they explore their relationship in a world that is both familiar and yet very different from the one they knew in the original story.
One idea could be that Harry starts having strange dreams where he sees through Voldemort's eyes. This could lead to him unknowingly helping Voldemort in some small ways at first, like revealing the location of some minor Order of the Phoenix safe houses. Another plot could involve the other characters slowly realizing something is wrong with Harry. For example, Hermione might notice that Harry is using some Dark Magic spells without hesitation, which is completely out of character for him.
A plot could involve a magical creature that bonds Harry and Voldemort in an unexpected way. For example, a rare faerie that makes them see each other's memories. Through this, Harry discovers some long - hidden secrets about Voldemort's past that could potentially change the course of their enmity. They then have to decide what to do with this new - found knowledge.
Yes, there are some fanfictions with this concept. It's an interesting take on the Harry Potter universe as it flips the typical relationship between Harry and Voldemort. In these fanfictions, the story often explores how Voldemort's actions change when he takes on the role of an adoptive figure to Harry, who has been abused. It can lead to some really unique character developments and plot twists.
One possible interesting plot point could be Harry's internal struggle. On one hand, he has been raised in an abusive situation, and on the other, he discovers this shocking relation to Voldemort. He might struggle with his own identity and whether he has the potential for evil within him like his supposed father.
One idea could be that Harry travels back in time and tries to prevent Voldemort's first rise to power. He could try to stop Voldemort from creating his first Horcrux, thus changing the entire course of the wizarding war. Another plot could be Voldemort traveling to the future to see how Harry defeated him, and then using that knowledge to change his past strategies. Maybe he would try to find a way to destroy the prophecy before it was made.