Well, a frequent theme is the expansion of the wizarding world. Since Harry loves books, he may come across books that describe new parts of the wizarding world not mentioned in the original series. Additionally, the theme of knowledge as power often appears. His love for books gives him an edge in understanding magic, and this can be used to his advantage in various situations. There can also be a theme of adventure, where the books he reads lead him on new and exciting quests.
Another theme is the isolation of the character. A 'dark child' Harry would likely be shunned or feared by others in the wizarding world. So, the fanfictions usually depict Harry being alone or having only a few allies who understand his darker nature. This can lead to a lot of internal conflict within Harry as he tries to find his place in a world that doesn't accept him. Also, the theme of redemption might be present. Even though he's a 'dark child', there could be a journey towards redemption as he realizes the consequences of his actions and tries to make amends, which can make for a very complex and engaging story.
One common theme is power and responsibility. Since Harry is not a child, he may have to face greater power struggles and more complex responsibilities in the wizarding world. Another could be relationships, like his romantic relationships or his more equal friendships with others as an adult.
One common theme is the corruption of innocence. Since Harry starts as a child, it shows how his initial pure and naive self can be influenced by the allure of dark magic. Another theme is the power struggle. As he becomes a 'dark lord', there are battles for control within the wizarding world.
One common theme is the adventure. Just like in the original works, characters from both universes often embark on grand adventures together. For example, Harry Potter characters might join the Fellowship of the Ring on their quest.
One common theme is friendship. Fanfictions often explore Harry's relationships with Ron and Hermione during their childhood years. Another theme is discovery of magic. Since Harry's a child, his first experiences with magic are often elaborated on.
One common theme is alternate universes. Fans like to imagine what would happen if the characters were in a different setting or if key events had changed. Another is the exploration of relationships. This could be romantic relationships between characters like Harry and Hermione, or friendships in more depth. Magic is also a huge theme, with new spells and magical concepts being introduced.
One common theme is the contrast between good and evil. Harry represents good, while Tom Riddle is the epitome of evil. Another theme could be the exploration of power. Tom Riddle is power - hungry and Harry has to learn to deal with his own emerging power. Also, there's often the theme of destiny. Harry is fated to face Voldemort (Tom Riddle), and this is often explored in fanfics, like how their paths cross in unexpected ways when Harry is a child.
One interesting thing could be that it shows Hermione's deep connection to the Harry Potter world even beyond the original books. It might also indicate that she enjoys exploring different interpretations and scenarios within the fandom.
Common themes in 'Harry Potter' fanfiction also include the backstories of minor characters. Writers often create detailed backstories for characters like Neville's family or Luna's early life. There are also fanfictions that revolve around the different houses in Hogwarts and their unique cultures. Some explore how students from different houses interact more deeply or how a character might change if they were sorted into a different house. Fanfiction gives a chance to expand on the rich world that J.K. Rowling created in the 'Harry Potter' series.
One common theme is alternate universes. Writers like to imagine what would happen if Harry was in a different magical school or if the wizarding world had different rules. Another theme is relationships, like exploring a romantic relationship between Harry and a different character than in the original books.