One possible plotline could be that Voldemort realizes that Harry is the key to some greater power or knowledge that he wants to obtain. So, instead of killing Harry, he saves him from another dark wizard or a dangerous magical creature. Maybe he sees that Harry has a connection to an ancient magic that could help him rule the wizarding world in a more complete way.
Perhaps Voldemort has a change of heart after seeing a vision of the future where Harry's death leads to the destruction of the wizarding world in a way he doesn't want. So he decides to save Harry. It could also be that there is a powerful magical bond between them that he discovers, and by saving Harry, he can use that bond to his advantage, like draining Harry's power gradually rather than killing him outright.
Another idea is that Voldemort discovers that there is a prophecy that if Harry dies by his hand, a far worse evil will be unleashed upon the wizarding world. So, in order to prevent this, he saves Harry. This could lead to a very strange alliance between them, with both having their own motives and distrust of each other, but being forced to work together to face the new threat.
One possible plotline could be that Voldemort discovers Harry's abuse through his Death Eaters' spying. He decides to save Harry as he wants to turn Harry to his side and believes that by rescuing him, Harry will be more likely to join him. Another could be that Voldemort has a moment of self - reflection and realizes that the way Harry is being treated is wrong, so he steps in.
One possible plotline could be that Harry discovers some sort of ancient magic or prophecy that binds him to Voldemort in a way that is not initially understood as a mate - like relationship. As they interact more, they start to see different sides of each other. Harry might try to change Voldemort, and Voldemort might find himself having unexpected feelings for Harry. Another plotline could involve a magical accident that fuses their magic in a way that creates this bond. Then they have to navigate their new relationship while dealing with the wizarding world's reactions.
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.
Perhaps in this fanfic, there has been some major plot twist that makes Harry believe that bowing to Voldemort is the only way to save his friends or the wizarding world. For example, Voldemort may have threatened to kill all of Harry's loved ones if he doesn't submit, and Harry is sacrificing his pride for the greater good.
One possible plotline could be that Harry realizes the wizarding world has some deep - seated flaws that Voldemort also wants to change. So, they form an alliance to reform the Ministry of Magic. For example, they might work together to end the pure - blood supremacy that has long plagued the wizarding society.
A possible plotline could be that Voldemort has a change of heart. Maybe he realizes that Harry has some power or knowledge that he needs, so he decides to carry Harry away to a secret location to extract that information from him.
One possible plotline could be a magical accident that causes Harry to be magically bound to Voldemort in a way that he has to 'ride' him, like being attached to his back or something. This could lead to a situation where Harry has to endure Voldemort's actions while trying to find a way to break free. And during this time, he might gain some insights into Voldemort's thoughts and plans, which he could later use against him.
A plotline could involve Petunia having a change of heart due to a magical event. For instance, she might accidentally stumble upon some of Harry's magic or receive a message from the wizarding world about his importance. This could lead her to stand up against Vernon and protect Harry. She could then start to teach Harry about his mother's side of the family and the magic that runs in his veins in a more positive way than the Dursleys ever did. And perhaps she even helps him develop his magic skills secretly before he goes to Hogwarts.
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.
One possible plotline is that Harry is magically transported to the Naruto world and discovers Naruto is in danger from a new, powerful enemy. Harry then uses his magic to fight alongside Naruto and his friends, saving Naruto in the process.