Maybe Harry discovers that Voldemort has a hidden motive for taking him as an apprentice. It could be that Voldemort believes Harry has a power that can be harnessed for a greater, more complex plan that involves not just ruling the wizarding world but changing the very fabric of magic itself. Harry then has to figure out how to either use this to his advantage or stop Voldemort from achieving his goal.
Another plot idea is that as Harry becomes Voldemort's apprentice, he starts to influence the Death Eaters. He gradually turns some of them against Voldemort by showing them a different path. This causes a rift within the dark side, and Harry has to balance his new - found power and influence while still trying to keep his true intentions hidden from Voldemort.
One idea could be an apprentice learning advanced potions from Snape. They might discover a new potion formula together. Another could be an apprentice in the art of defensive magic under Moody. They could learn unique spells and how to counter dark magic attacks.
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 plot idea might be that Snape, in taking Harry as an apprentice, is actually trying to atone for his past mistakes. He trains Harry in a very strict but effective way, and they end up bonding over their shared experiences with Voldemort. Harry could learn how to better control his emotions and magic from Snape, which helps him in future battles.
One idea could be a young wizard from Hogwarts being secretly recruited by a Sith Lord. Maybe they discover a hidden connection between the Dark Side of the Force and some of the darkest magic in the wizarding world, like the Unforgivable Curses. The wizard, as a Sith apprentice, could then start using Sith powers alongside magic, creating a whole new kind of chaos in both the Star Wars and Harry Potter universes.
Well, an interesting plot could be that the apprentice is sent on a secret mission by Grindelwald. On this mission, they encounter a magical creature that has the power to change loyalties. This makes the apprentice question their place by Grindelwald's side. Also, the apprentice might find a hidden diary of an old wizard who was once in a similar position, and through reading it, they start to see the flaws in Grindelwald's plans.
One idea could be that the apprentice discovers a new type of wood with extraordinary magical properties. For example, a wood that can channel the magic of multiple elements at once. This could lead to a race among dark wizards to get their hands on wands made from this wood, and the apprentice has to protect the secret of its origin.
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.
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.