Perhaps they are mates but a dark curse has made Voldemort forget this. Harry has to go on a quest to find a way to break the curse. Along the way, he discovers secrets about the past that change the way he views Voldemort. He also has to face the opposition of his friends who can't understand why he would want to save Voldemort.
One plot idea could be that they discover they are mates due to a powerful magical bond. They then have to hide this fact from the wizarding world which is in the midst of a war. Harry might try to convince Voldemort to abandon his dark ways, using their newfound connection as a way to appeal to his better nature.
Another plot could be that they are mates and it was all part of a prophecy that no one understood correctly. Their relationship starts to change the balance of power in the wizarding world. Some magical creatures start to side with them because they believe their union is fated. But they face a lot of challenges from the Death Eaters who don't want Voldemort to change and from the Order of the Phoenix who don't trust Voldemort.
One idea could be that Harry, as Voldemort's soulmate, tries to use their connection to redeem Voldemort. He might show Voldemort memories of love and kindness in an attempt to change his dark ways.
Well, perhaps in this fanfic, Harry is raised by Voldemort to be his ultimate weapon against the wizarding world. He's trained to be extremely powerful in combat magic. But as he grows, he starts to notice the fear and suffering Voldemort causes. This could lead to an internal struggle within Harry, between his loyalty to the man who raised him and his own sense of right and wrong. Also, there could be a plot where Harry uses his unique position to secretly undermine Voldemort's plans from within his own ranks.
A simple plot idea is that the Boggart - Voldemort becomes so powerful in its manifestation that it starts to physically interact with the real world. Harry has to figure out how to stop it before it causes real damage. Another idea is that the Boggart - Voldemort begins to target Harry's friends one by one, using their worst fears against them. Harry then has to find the strength to face his own Boggart - Voldemort to save his friends. Also, what if the Boggart - Voldemort was actually being controlled by a new, hidden enemy? Harry would have to uncover this mystery while also dealing with his fear of the Dark Lord.
Well, a possible plot could be that Harry is cursed in a way that slowly changes his values and beliefs. As he starts to side with Voldemort, he begins to enforce new, dark rules in Hogwarts. The other students are in shock and disbelief. Some of his friends like Hermione might try to break the curse, while others like Ron might be more quick to condemn him. Also, Voldemort could use Harry as a tool to gain more support from the wizarding community, as Harry was once seen as the 'Chosen One' to defeat him. So with Harry on his side, Voldemort could deceive many into thinking his ways are not so bad.
One interesting plot could be that they are mates due to a powerful ancient magic that was long forgotten. This magic forces them to understand each other's pasts and they start to question their long - held enmity. For example, they might discover that they both had difficult childhoods which led them to their current paths, and this newfound understanding makes them consider a different future together.
One idea could be that Harry and Lucius are forced to work together on a secret mission for the wizarding world. Along the way, they start to see each other in a new light and gradually form a bond. Another could be that they are both victims of a dark magic spell that makes them share emotions, leading to an unexpected connection. Maybe they discover a long - lost prophecy that ties their fates together and they must figure out how to fulfill it while dealing with their initial animosity.
One idea could be that they go on a secret mission together. Maybe they have to uncover a hidden magic that is causing problems in the wizarding world. Another could be that they bond over their difficult pasts, like Harry's with the Dursleys and Remus' as a werewolf. They could support each other through tough times at Hogwarts, like when dealing with the prejudice some students have towards Remus because of his condition.
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.