Another theme is redemption. Sometimes the oc tries to redeem Voldemort or is on a journey of self - redemption while being associated with him. They might see some good in him that no one else does and strive to bring it out. It adds a lot of depth to the story. And also, the theme of survival is quite prevalent. The oc has to survive in the dangerous world dominated by Voldemort and his Death Eaters, constantly facing threats and having to outwit their enemies.
Power is a common theme. The oc often has to deal with Voldemort's thirst for power and either resist it or be drawn into it. For example, in many fics, the oc discovers a new form of magic that Voldemort wants to use for his own evil purposes.
One common theme is redemption. The OC son of Voldemort often tries to atone for his father's sins. Another is identity. He struggles to figure out who he is outside of being Voldemort's son.
It might be considered somewhat common. Since Voldemort is such an iconic and complex villain in the Harry Potter series, adding an oc to his fanfiction gives writers the chance to explore different relationships and storylines. For example, an oc could be a love interest for one of Voldemort's followers, or a secret weapon against him. However, it's not the most dominant trope, but still quite popular among those who write fanfiction in this genre.
A prevalent theme is the dark nature of their relationship. It's often depicted as being based on a mutual love for the dark arts and a desire for domination. In some fanfictions, there's also the theme of obsession. Bellatrix's obsession with Voldemort is a central part, like in the fanfiction 'The Darkest Devotion' where her every action is centered around pleasing and serving Voldemort. And of course, the theme of fear also plays a role, as many characters in the wizarding world fear their combined might.
Common themes include the exploration of dark magic. If Harry is Voldemort, he would be more involved with dark magic, so the fanfictions often show his journey into that world. Also, the theme of redemption might be present. Even though he's in the form of Voldemort, he might try to atone for Voldemort's past deeds. And there's always the theme of how the wizarding world reacts to this strange transformation of Harry.
One common theme is loyalty. Bellatrix is extremely loyal to Voldemort in the original series, and this is often explored more deeply in fanfiction. For example, stories might show her unwavering commitment to him even in the face of great danger.
Love or obsession is also a frequent theme. Sometimes, there's an unexpected love story between them, which is quite a contrast to their original characterizations. Other times, it's more of an obsession on Voldemort's part towards Bella. This can lead to complex relationships and storylines that keep readers hooked as they try to understand the nature of this unusual connection.
The theme of the dark side of magic can also be prevalent. Since both are dark wizards, fanfictions may explore their use of dark magic together. It could include their experiments with curses, or how they plan to use dark magic to achieve their goals, like taking over the wizarding world. Their relationship might be intertwined with their shared love for the most dangerous and forbidden forms of magic.
A frequent theme is the exploration of a different kind of relationship. Since Voldemort is now female, it could lead to a more complex emotional and psychological relationship with Harry. Also, the theme of identity is common. How does a female Voldemort view herself compared to her male counterpart? And how does Harry react to this new version of his arch - enemy? These are often explored in such fanfiction.
Another theme is the exploration of their relationship outside of the typical hero - villain dynamic. Some fanfictions show that there could be a more complex emotional bond between them. Maybe it's a begrudging respect or a strange attraction that they can't fully explain. This adds a whole new layer to the story.
One common theme is redemption. The idea that Voldemort can be saved or changed through Harry's help. Another theme is the exploration of power dynamics. Since Harry and Voldemort have very different views on power, seeing how they interact when Harry is trying to help can be really interesting.