The theme of identity is often explored. She has to figure out who she is in relation to her father, the Black family, and the wizarding world at large. There's also the theme of friendship. In many fanfictions, she forms bonds with other characters that help her navigate through the difficulties. For example, she might befriend characters who knew her father or are also dealing with their own family issues. And power, both magical and political, can be a theme as the Blacks were a powerful family.
In addition to the above, the theme of love can play a significant role. It could be love for her family, even with all their flaws, or romantic love. Her love might be what drives her to uncover the truth about her father or to protect the family name. Also, the struggle against dark magic is a theme. Given the Black family's association with the dark side in the past, she may have to fight against those who still uphold the old, dark ways.
One common theme is family legacy. Since she is the daughter of Regulus Black, there's always the exploration of what it means to carry on the Black family name and all the baggage that comes with it, like the family's dark past and their pure - blood ideology.
Isolation is a common theme. In many fanfictions, Regulus is shown to be alone in his struggles, with no one to truly understand what he's going through. His family's focus on pure - blood ideals and dark magic often makes him feel like an outcast even within his own home.
One common theme is his isolation. Regulus often feels alone, whether it's because of his family's secrets or his own actions. Another theme is regret. He regrets getting involved in the Dark Arts and wishes he could have made different choices.
Family dynamics is also a big theme. The Black family has its own set of rules and expectations, and Regulus being trans can create a lot of tension within the family. In some fanfictions, we see how his family reacts to his new identity, whether it's rejection or eventually acceptance.
Love and sacrifice are also often themes. The lemon aspects can sometimes be related to a passionate love story, and sacrifice is something that was part of Regulus's character in the original lore. He sacrificed himself for what he believed was right.
Friendship or an unlikely alliance is also a common theme. Harry and Regulus come from different generations but they share a connection through the Black family. Fanfictions like to explore how they form a bond, whether it's a friendship based on mutual understanding or a strategic alliance to defeat a common enemy.
One common theme is sacrifice. In many fanfictions, Regulus Black's death is seen as a selfless act for a greater good. Another theme is redemption. His actions are often portrayed as a way to make up for past mistakes.
One common theme is forbidden love. Given Regulus's background in a pure - blood family and his association with the Dark Arts initially, a relationship with someone from a different world, like a Muggle - born or someone on the side of the Order of the Phoenix, is often explored. For example, in many fics, he might fall for a Gryffindor which goes against the traditional Slytherin - Gryffindor rivalry.
One common theme is redemption. Regulus is often depicted as seeking redemption for his past actions in the Death Eaters, and the OC can play a role in that journey. For example, in some stories, the OC helps him see a different path.
One common theme is identity. Fem Harry has to come to terms with being Regulus' daughter and how that changes her perception of herself. Another is family legacy, as she has to deal with the Black family's reputation.