One plotline could be that the de - aged Voldemort, now in a more innocent state, tries to make amends for his past actions towards Harry. He might use his knowledge of magic to protect Harry from other threats in the wizarding world. Another could be that Harry initially resists the idea of Voldemort as a father - figure but slowly comes to understand that this new Voldemort is different. And then there's the plotline where the wizarding world is in chaos because of this strange relationship, with some people believing it's a dark plot while others see it as a chance for redemption.
In a 'deaged Harry Voldemort father fanfiction', a plotline could revolve around the mystery of how Voldemort was de - aged. Harry might be the one to discover the truth behind it, which could be linked to some ancient magic. As he tries to understand this, he also has to deal with the growing relationship with Voldemort. Voldemort, on the other hand, could be struggling with his new identity and the feelings he has towards Harry. Maybe he starts to have paternal instincts that he doesn't understand at first. The wizarding world might be divided over this situation. Some might think Harry is being corrupted, while others believe it could be the key to a new era of peace in the wizarding world. This could lead to a lot of conflict and drama as Harry and Voldemort navigate their relationship while also facing the opinions and actions of the rest of the wizarding community.
One possible plotline could be that Harry is tricked by Voldemort. Voldemort might use some powerful dark magic or false promises to make Harry believe that joining him is the right thing to do. For example, Voldemort could claim to have a way to end all the bloodshed in the wizarding world if Harry helps 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.
One possible plotline could be that Harry is under some sort of dark magic influence or curse that forces him to aid in Voldemort's resurrection. Maybe a powerful dark wizard has manipulated Harry's mind.
Another plotline might be their family life. They could be shown starting a family early, and dealing with the joys and challenges of raising children in the wizarding world. For example, teaching their kids magic at home, or dealing with the chaos of a family full of young wizards.
One possible plotline could be that Voldemort discovers Harry's abuse through his Death Eaters' spying. He decides to save Harry as he wants to turn Harry to his side and believes that by rescuing him, Harry will be more likely to join him. Another could be that Voldemort has a moment of self - reflection and realizes that the way Harry is being treated is wrong, so he steps in.
One common theme is redemption. With Voldemort deaged, there's often a chance for him to be redeemed or at least have a different path than his original evil one. For example, in some fanfictions, Harry becomes a sort of mentor to the deaged Voldemort, guiding him towards good.
Perhaps Voldemort discovers a long - lost family secret of Harry's that gives him leverage. For example, he might find out that there is a curse on Harry's family that only he can break, but only if Harry joins him. Or Voldemort could be using some form of mind - control device that was created by an ancient dark wizard, which he uses on Harry to make him a part of his ranks.
One possible plotline could be that Harry has to hide this fact from his friends at Hogwarts, fearing their reaction. He might struggle with feelings of guilt and confusion, especially if he starts to show some of Voldemort's darker tendencies.
One plotline could be that Voldemort wants to use the marriage to gain control over Harry's special powers. Another could be that it's part of a prophecy that neither of them can avoid. Or maybe it's a political move in the wizarding world to end the war between the two sides.
One possible plotline could be Harry trying to find a way to break the forced marriage bond while secretly gathering allies to fight Voldemort.