Well, a frequent theme is the exploration of opposites. Harry is the symbol of good and Voldemort of evil in the original story. In yaoi fanfiction, writers often play with this dichotomy, seeing how these two extreme characters could come together. Also, there's often a theme of secrecy and forbidden love, as their relationship would be frowned upon in the wizarding world.
Power is a common theme. Fanfictions often explore how Voldemort amasses and uses his power, and how others react to it. For example, some stories might show characters trying to resist his power grab.
Well, a major theme is the rediscovery of magic. In the original series, a lot of magic was used in the fight against Voldemort. In these fanfictions, magic can be used for more positive, creative things. Also, friendship is often explored in a different light. Without the dark shadow of Voldemort, friendships can form and develop based on different interests and personalities rather than just being part of a resistance against the Dark Lord.
In 'Harry Potter Lord Voldemort fanfiction', the theme of manipulation is quite common. Voldemort is a master manipulator in the books, and fanfictions show how he manipulates others to do his bidding. The theme of prophecy also appears often. Since the prophecy about him and Harry was important in the original series, fanfictions may play around with different prophecies related to Voldemort. And the theme of fear is there too. Voldemort instills fear, and fanfictions might explore how that fear affects the wizarding world in different ways.
One common theme is the power struggle. Since they are on opposite sides in the original story, their relationship as mates can create a complex power dynamic. Another is the idea of redemption for Voldemort.
One common theme is power. Voldemort is always associated with his quest for power, and when his snake aspect is involved, it often symbolizes another layer of his power - something more primal and dangerous. For example, in many fanfictions, his control over Nagini is seen as a key part of his power structure.
Love and hate are also common. The complex relationship between fem Harry and Voldemort can be filled with both love and hate. It could be a romantic kind of love or a more platonic love - hate relationship. Identity is another theme. Fem Harry may have a different identity in the wizarding world compared to male Harry, and this affects how she deals with Voldemort.
One common theme is power struggle. Harry, as a slave, tries to resist Voldemort's power. Another theme is the exploration of the dark side. It shows how the dark magic and Voldemort's influence can change Harry's situation. Also, the theme of redemption might be there, like Harry trying to find a way to break free and still uphold good in the end.
A very frequent theme is the exploration of destiny. The fanfictions often question if their being soulmates was fated all along. Also, the theme of internal conflict is common. Harry has to grapple with the fact that his soulmate is the most evil wizard, and Voldemort has to face his connection to the 'Boy Who Lived'. It makes for some really intense character development. For example, in some stories, Harry tries to find the good in Voldemort because of their bond, while Voldemort is forced to re - evaluate his hatred for Harry. This leads to a lot of interesting plot twists and turns.
A frequent theme is the breaking of boundaries. In the original story, Harry and Voldemort are on opposite sides of a huge divide. Fanfictions with this plot explore what happens when those boundaries are shattered. Also, there can be a theme of discovery. Harry discovers a new side to Voldemort that no one else has seen, and this leads to their relationship. And of course, there's the theme of transformation, both for Voldemort in terms of his character and for the whole 'Harry Potter' world view as a result of their relationship.
One common theme is redemption. It often involves Voldemort getting a chance to redeem himself through his friendship with Harry. Another theme could be the exploration of power. How would their combined powers be used? Also, the theme of forgiveness might be present as Harry has to forgive Voldemort's past actions for the friendship to work.