Family acceptance is another theme. Harry has always been an orphan, so in these fanfictions, when he gets a father figure, there's a lot about him learning to accept this new family member and vice versa. It's about building that bond of family that Harry has always longed for.
Growth and protection are also common. The father figure usually helps Harry grow as a wizard and a person, and at the same time, protects him from the various threats in the wizarding world. This could be from dark wizards or just the general hardships of growing up at Hogwarts.
One common theme is responsibility. Harry has to quickly learn how to be responsible for another life. Another theme is growth. He has to grow up faster than his peers to take care of his child.
One common theme is sacrifice. Harry often has to sacrifice his own desires or time for his child. For instance, he might give up on a dangerous but exciting adventure to be there for his kid's first day at Hogwarts.
Well, there's often a theme of protection. Since Sirius is Harry's father, he would be extremely protective. Also, the theme of identity is prominent. Harry's understanding of himself and his place in the wizarding world would change significantly. There are also themes of love and sacrifice that are reimagined in the context of this new relationship. For instance, Sirius might sacrifice even more for Harry knowing he is his son.
One common theme is redemption. Snape, who had a troubled past with Harry, gets a chance to redeem himself through the father - son relationship. Another theme is the discovery of hidden family ties. Harry, who has always thought he was an orphan, now has to come to terms with Snape as his father.
One common theme is the clash and then the fusion of two different worlds - the magical world of Harry and the technological world of Tony. Another theme could be the father - son relationship development, showing how they learn to understand and support each other.
One common theme is redemption. Snape gets a chance to be a better person through being Harry's father. Another is the exploration of family. It shows how family isn't just about blood but also about the bonds you form.
One common theme is redemption. The idea that Harry could be Tom's father might lead to a path of redemption for Tom Riddle. Another theme could be alternate history. Since this is a big change to the original story, it often explores how different the wizarding world would be.
The theme of discovery is big. Harry discovers more about his father's true nature, his friends, and his adventures. Another common theme is the re - imagining of Harry's family life. Instead of growing up with the Dursleys, he might have a completely different family environment with his father around. And of course, there's the theme of love, as the father - son bond is explored in different and often heart - warming ways.
One common theme is redemption. Sirius' father may have been a strict or even cruel figure in the original story, but in fanfiction, he could be seeking redemption for his past actions towards Sirius. Another theme is legacy. How does the Black family legacy affect the relationship between father and son? It could be a burden or a source of pride.
The theme of sacrifice is also quite prevalent. In many fanfictions, Merlin's father has made sacrifices for Merlin's future, which Merlin discovers as the story progresses. It gives a deeper understanding of their relationship.