A really cool plot could be that time travel causes a paradox. For example, Harry travels back in time to stop Voldemort from being born. But in doing so, he creates a situation where he himself is never born. Then, the story could focus on how Harry tries to fix this paradox. He might have to work with some powerful magic or even some ancient magical creatures to set things right. And all the while, Voldemort could be aware of the changes in time and be trying to take advantage of the chaos.
Well, a plot might involve Harry and Voldemort both traveling back to a time when the founders of Hogwarts were alive. They could each try to gain the support of the founders. Harry might seek help from Gryffindor and Ravenclaw, while Voldemort would be drawn to Slytherin. This could lead to a great battle of wits and magic among them and the founders, with the future of the wizarding world at stake. It could also explore how the Hogwarts houses were originally influenced by such a conflict.
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.
One interesting 'Harry Voldemort Time Travel Fanfiction' story could be where Harry travels back in time and tries to prevent Voldemort's rise to power in a more covert way. Maybe he disguises himself and subtly changes events that led to Voldemort's initial gain of power.
Another plot might involve Voldemort traveling to the future to see the destruction he causes. This could lead to a moment of self - reflection for him. He might realize the futility of his actions and perhaps change his ways. It would be a fascinating exploration of his character.
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.
A possible plot is that Harry and Tonks time travel to a point where the creation of Horcruxes was just beginning. They could try to stop Voldemort from splitting his soul, which would change the whole course of the wizarding world's history. They would face many challenges, like not being discovered by Voldemort or his followers. Also, Tonks might use her skills to distract or mislead some of the Death Eaters while Harry looks for a way to destroy the Horcrux - making materials or the places where Voldemort hid them.
A great plot might be that Teddy time travels to a time when Harry was a child living with the Dursleys. Teddy could try to secretly protect Harry from some of the bullying and neglect he endured. Or perhaps Harry and Teddy travel to a time long before Hogwarts was founded to learn about the origins of magic in the wizarding world, and they bring that knowledge back to their own time to help improve modern magic.
A more complex plot might be where both Harry and Tom are sent back in time to a point where the Hogwarts founders are still alive. They get involved in the founding events of Hogwarts, and Harry has to stop Tom from influencing the founders in a way that would make Hogwarts more exclusive and dark - sided. Harry could try to work with the founders to see the potential danger in Tom even at a young age.
Perhaps they travel back to a time in Hogwarts' history. Harry could interact with his parents as a student, and Narcissa could see a different side of the school. They might end up changing the way certain relationships develop. For instance, Harry could influence his father to be less of a bully, which in turn could have a huge impact on the future friendships and enmities at Hogwarts.
Another plot idea is that Harry Peverell time - travels to different points in his own family's history. He gets to meet his ancestors, learn about their secrets and magic, and discovers that his family has a much larger role in the wizarding world's history than he ever thought. He could then use this knowledge to better understand his own place in the wizarding world and his connection to the Deathly Hallows.