One possible plotline could be Harry struggling to regain control of his mind while Voldemort tries to use his body to carry out evil deeds. For example, Harry might have to fight against Voldemort's commands internally. Another could be how his friends notice the strange behavior and try to figure out what's wrong and save him.
One interesting plotline could be an alternate universe where Harry and Voldemort have to team up against a greater evil. This flips the traditional dynamic on its head.
One interesting plotline could be exploring Voldemort's family history before he became the Dark Lord. Maybe there are secrets about his parents that were hidden. For example, in some fanfictions, they might delve into how his mother's family had some sort of ancient magic that was passed down but misused by Voldemort. Another could be about Voldemort having a long - lost relative who tries to stop him. This relative might have been hidden away all this time and has now emerged with unique powers to oppose Voldemort's tyranny.
One interesting plotline could be Voldemort being redeemed. It's a really different take as in the original books he's pure evil. But in fanfic, some writers explore what if he had a change of heart.
In such fanfiction, a common plotline is that Harry's decision to join Voldemort creates a power shift. He might use his status as the 'Boy Who Lived' to undermine Dumbledore's reputation further. Since many in the wizarding world had put their faith in Harry, his joining Voldemort could lead to widespread panic and confusion. Harry could also start to recruit other disillusioned wizards to Voldemort's cause, using his own charm and the false narrative that Voldemort is actually a liberator of the wizarding world from the old, restrictive ways.
Well, in some of these fanfictions, Harry's magical abilities might change when possessed. He could start using dark magic in ways he never would have before. It makes for a really interesting twist on his character. For example, he might struggle against Voldemort's will while still being forced to do some dark deeds, creating a complex internal battle.
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.
One possible plotline could be the daughter of Voldemort struggling with her identity. She might be torn between her father's dark legacy and the potential for good that she sees in the wizarding world. Another could be her forming an unexpected alliance with Harry Potter, perhaps to atone for her father's sins or to find a new path for herself.
One interesting plotline could be that Harry discovers a new, powerful magic that gives him an edge in the fight against Voldemort. For example, he might find a way to combine different spells in an unexpected way. Another could be that Voldemort has a secret weakness that Harry uncovers through his friendships. Maybe one of his friends has a unique connection to an ancient magic that can be used against Voldemort.
One possible plotline could be that the children possess unique magical abilities due to their mixed heritage. For example, they might have the power to control both light and dark magic in a balanced way. Another could be that they are caught in the middle of the two worlds, with the wizarding community being suspicious of them because of their Voldemort connection, while Voldemort's remaining followers might try to use them for their own dark purposes.
Well, in some fanfictions, when Harry surrenders to Voldemort, it leads to a strange alliance. Voldemort, being overconfident, might keep Harry alive thinking he can use him. Harry could then start to learn more about the dark side from within, and perhaps find a way to turn some of Voldemort's followers against him. Also, it could be that the act of surrender causes a shift in the magical balance in the wizarding world. Some ancient prophecies might be set in motion that no one expected, and Harry has to figure out how to use this new situation to his advantage.