Well, there's often the theme of isolation too. Even though he has people around him, Sirius can feel alone in his sickness. He might be in his own head, thinking about his past traumas and how his current state is just another hurdle. And then there's the theme of discovery, like finding out what's really wrong with him and how to fix it, which drives the plot forward in many of these fanfictions.
One common theme is the vulnerability of Sirius. Usually, he's portrayed as this strong, brave character, but when sick, he shows a different side. Another theme is the support from his friends and family. They often rally around him, highlighting the importance of those relationships.
A lot of these fanfictions explore the idea of mortality. Since Sirius has had a difficult life, his illness can make readers think about his possible death. Also, the theme of recovery and hope is present. The characters around him try their best to nurse him back to health, giving a sense of optimism.
The theme of freedom is quite important too. Sirius was locked up in Azkaban for a long time, and when he escaped, he was fighting for his freedom. Fanfictions often expand on this, showing his journey to fully regain his freedom and his place in the wizarding world, while also dealing with the prejudice against him because of his supposed crimes.
The theme of redemption also shows up. Since Sirius had a somewhat 'tainted' reputation in the eyes of the Black family, some fanfictions explore if Orion could have played a role in his redemption or if Sirius could have redeemed the family name in Orion's eyes.
One common theme is family conflict. Sirius rebels against the pure - blood mania and dark magic - associated ways of the Black family that Walburga represents. Another theme is the struggle for identity. Sirius is trying to be his own person away from the expectations set by his family with Walburga as a symbol of those family constraints.
One common theme is love. Often, the female character falls in love with Sirius Black, exploring the different aspects of romantic relationships in the wizarding world. For example, how they deal with the danger he's in and the secrecy required in their relationship.
One common theme is loyalty. Sirius was extremely loyal to his friends, and fanfictions often explore this aspect in different situations, like when he was in Azkaban but still held onto his love for James and his friends.
One common theme is unrequited love. Sirius might be jealous because he loves someone who doesn't seem to return his feelings in the same way. For example, he could be jealous of someone else getting closer to the person he has feelings for.
A prevalent theme is the exploration of relationships. His relationships with his friends, like Remus and James (in flashbacks or through memories), are re - examined. How they support or sometimes struggle to support him is a big part of these fanfictions. Also, the relationship with the potential father of the child (if it's part of the story) is often a central focus, full of drama, love, and sometimes conflict.
One common theme is family. Since Sirius comes from the Black family, stories often explore how his child is related to the family's complex history, traditions, and dark secrets.
One common theme is redemption. Since Sirius had a troubled past, many fanfictions focus on him trying to make up for his past mistakes as a professor. Another theme is family. He might be a father figure to some students, especially those who lack proper family support at Hogwarts.
One common theme is family heritage. Since the Blacks are an old pure - blood family, many fanfictions explore how Sirius Black's mother dealt with the family's traditions and expectations. Another theme is the mother - son relationship. Given Sirius's rebellious nature, how his mother reacted to it and how it affected their relationship is often explored.