A plotline could start with Harry having strange dreams or visions about Voldemort that are not of the typical enemy - related kind. As he investigates, he realizes there is a deeper connection. Maybe there is a hidden spell or curse that was placed on them long ago. As they come together, they face challenges from both the Order of the Phoenix and the Death Eaters. Harry could use his influence on Voldemort to try and stop the dark magic he's been spreading, and Voldemort could use his power to protect Harry from those who want to harm him because of their relationship.
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.
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.
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.
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.
I'm not sure if it's a popular fanfic concept. There are so many different types of fanfics out there. While the idea of Harry Potter being Voldemort's mate is very different from the original story, it might attract some readers who are looking for something completely new and unexpected. But it could also turn off a lot of fans who prefer to stick to the more traditional interpretations of the characters.
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 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 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.
Perhaps Voldemort uses magic to brainwash Harry into thinking he is his pet. Harry then has to fight against this false identity while still being under Voldemort's control. The plot could involve Harry slowly regaining his true self and using his new - found knowledge of Voldemort's inner workings (from being so close to him) to defeat him in the end. It would be a journey of self - discovery and resistance.
One possible plotline could be that Voldemort captures Harry early on and instead of killing him, decides to keep him as a pet. Harry, being in this strange situation, might start to develop a warped sense of loyalty to Voldemort over time.
One possible plotline could be Harry coming to power after a great war in the wizarding world. He then has to deal with rebuilding and unifying the different wizarding communities under his rule as emperor. He might face resistance from some traditional wizarding families who don't want to accept a new form of government.