Another possibility is that it's an accidental result of a powerful magic clash between Voldemort and Harry. When their magic collides in a certain way, it causes an unexpected transformation on Harry, turning him into a girl. This would add a whole new layer of complexity to their relationship and the overall story.
One way it could be plausible is through the use of dark magic manipulation. Voldemort is a master of dark magic. He could perhaps use some form of mind - altering spell on Harry. If he managed to get Harry in a vulnerable state, like when he was feeling isolated or betrayed, the dark magic could seep in and start to change his nature. For example, if Voldemort made Harry believe that his friends at Hogwarts had turned against him, Harry's sense of justice and goodness might be warped, leading him to the dark side.
The exploration of the dark side of magic is also a theme. We get to see how the dark side can be so alluring. In these fanfictions, Harry may start to explore spells and powers that he would never have considered before. And the theme of betrayal is often there. Harry might feel betrayed by the wizarding world that he once fought for, which makes it easier for Voldemort to turn him.
The deviation from the original story makes it unique. In the original 'Harry Potter' series, Harry is the hero against Voldemort. But in these fanfics, the roles are being reversed in a sense, which is a huge departure from the norm.
There could be a fanfic where Harry is tricked into believing that Voldemort is the only one who can save the wizarding world from a greater threat. For example, a new, more powerful dark force emerges and Voldemort convinces Harry that they need to join forces. In this type of fanfic, Harry would gradually turn towards Voldemort's side as he is deceived by false promises.
One consequence could be the change in Harry's relationships. His friends might initially be shocked and have to adjust to this new version of Harry. For example, Ron might struggle with his feelings towards Harry now that he's a girl.
One possible plot could be that the magic backfires on Voldemort during a powerful dark spell. As a baby, he has no memory of his previous self. The wizarding world is in chaos as they don't know what to do with this baby - voldemort. Some might want to kill him immediately, while others think he could be redeemed. Harry Potter, having defeated him once, feels a strange sense of responsibility and wants to make sure he doesn't grow up to be evil again.
Perhaps the plot could involve Petunia having some sort of magical awakening or discovery that leads her to perform this transformation on Harry. In this fanfiction, Harry might then use her new female identity to infiltrate parts of the wizarding world that are more difficult for a male to access, like certain all - female magical groups. And this could lead to all sorts of adventures and revelations about the wizarding world's gender dynamics.
Obviously, Voldemort as a baby would be one of the main characters. Harry Potter could also be a main character as he has a long - standing enmity with Voldemort. He might be involved in deciding the fate of the baby - voldemort. Dumbledore would surely play a major role. His wisdom would be needed to deal with this strange situation. And Hermione, with her vast knowledge of magic, could be involved in trying to find a way to either reverse the transformation or ensure that the baby Voldemort grows up to be good.
Petunia's character could also develop in this fanfiction. If she turns Harry into a girl, she might start to feel guilty or responsible. This could lead to a change in her relationship with Harry. Maybe she would start to see Harry as more of a person rather than just a burden. And she could also develop some magical abilities further as she has now performed such a complex spell on Harry.