A big theme is the discovery of self - reliance. Harry, who is used to having some support at Hogwarts, now has to rely on his own instincts and Sirius' guidance. Also, the theme of secrecy plays a part. They need to keep their presence on the streets a secret from certain people in the wizarding world who might try to use their situation against them. This adds an element of mystery and danger to the story.
The theme of adaptation is very prominent. Harry has to adapt to a completely different lifestyle from what he's known at Hogwarts. He has to get used to the dirt, the cold, and the lack of regular meals. Sirius, on the other hand, has to adapt to having Harry with him and being responsible for him in this new, difficult situation. There's also the theme of hope, as they both look forward to a better future and getting out of their current situation.
One common theme is friendship. Sirius often uses his friendship with Harry as a starting point. He knows Harry well and wants to see him happy, so he uses that knowledge to bring Harry and Ginny closer. Another theme is the power of the past. Sirius, being from an older generation, has a different perspective on relationships. His influence from the past helps shape the relationship between Harry and Ginny in the present.
The themes often include love and protection. Remus and Sirius would be very protective of Harry, and this love and protection would shape Harry's character. Another theme is the struggle against prejudice. Given that Remus is a werewolf, there could be prejudice that Harry has to face because of his family. Also, the theme of self - discovery is common, as Harry discovers his place in this new family and in the wizarding world.
One common theme is redemption. Since Regulus was once a Death Eater, many fanfictions focus on him trying to make up for his past wrongs. For example, he might help the Order of the Phoenix in secret or try to protect innocent wizards.
One common theme is family. Sirius is Harry's godfather, so many fanfictions explore the idea of family that goes beyond blood. Another theme is redemption. Sirius has a past full of mistakes and he often tries to redeem himself through his relationship with Harry. Also, protection is a big theme. Sirius is always trying to protect Harry from the threats in the wizarding world.
Another theme could be Sirius helping Harry deal with the difficulties at Hogwarts. This might include protecting Harry from bullies like Draco Malfoy or guiding him through the challenges of certain classes. In some fanfictions, Sirius might use his experience as a Gryffindor to inspire Harry and give him confidence.
One common theme is family. These fanfictions often explore the idea of what a real family could be for Harry, with Sirius as a loving father figure. Another theme is the exploration of Sirius's character. Writers like to show how he overcomes his past traumas to be a good parent to Harry.
Another theme is redemption. Sirius may feel he has a lot to make up for, especially considering his past mistakes. By being protective of Harry, he's sort of redeeming himself. For example, in many fanfics, he tries to prevent Harry from making the same mistakes he did when he was younger.
In these fanfictions, a recurring theme is the change in their relationship. The pregnancy can bring them closer together or create new tensions. Also, the theme of protecting the unborn child from potential threats in the wizarding world, whether it's dark wizards or the prejudiced views of others, is quite common. Additionally, there may be exploration of how Hermione's academic life is affected and how Sirius adapts to the idea of becoming a father, which are all interesting aspects that authors like to explore.
A big theme is the exploration of Hermione's character growth. With Sirius as her guardian, she might be exposed to different aspects of magic and life that she wouldn't have otherwise. For example, she could learn more about the darker side of magic and how to combat it. Also, the theme of loyalty often comes up, as Sirius is loyal to Hermione and she to him in return.
One common theme is mentorship. Sirius often acts as a mentor to Harry, guiding him in the wizarding world. Another is family. Since Sirius is like family to Harry, the fics explore the concept of a non - traditional family. And there's also the theme of adventure as they usually go on some sort of adventure together.