I'm not sure if there is such a specific fan fiction. Fan fiction can cover a vast range of wild and creative ideas, but this one seems rather unusual and might not be a common theme. However, the Harry Potter fan fiction community is huge, so it's possible that something like this exists in some corner of the internet.
The themes often revolve around protection. Sirius was always protective of Harry, so that's a big part of these fan fictions. Also, the theme of self - discovery for Harry, as Sirius helps him understand his identity as a wizard and as a member of the Potter family. Additionally, friendship is a theme as their relationship is a special kind of friendship that goes beyond normal wizard - wizard relationships.
Well, I've read 'A New Chapter with Sirius and Harry'. In this fan fiction, Sirius manages to come back to life in a rather unique way. The story then unfolds as he and Harry team up to fight new threats in the wizarding world. Another good one is 'Sirius's Legacy for Harry' which emphasizes how Sirius's actions and beliefs have a long - lasting impact on Harry's journey as a wizard.
A 'sirius gets harry pregnant' fan fiction would be quite a complex and controversial piece. It would need to establish a new relationship dynamic between Sirius and Harry that was not present in the original series. The story could involve how they deal with the news of the pregnancy, the reactions of other characters in the wizarding world, and how they navigate their new situation while still dealing with the threats and challenges that exist in the Harry Potter universe. It would be a very different take on the characters and their lives, and would likely attract a lot of mixed reactions from fans.
Harry's character grows in terms of self - confidence. Knowing that Sirius is his father gives him a stronger sense of identity. He no longer feels like an orphan in the truest sense. He becomes more assertive in the wizarding world, demanding respect and answers. For example, he might be more forceful in his interactions with Snape, not tolerating any slights as he now has a powerful family connection.
Well, if such a fanfiction exists, it's really disturbing. It goes against the positive relationship we expect between Sirius and Harry in the original Potter universe. It might make many fans uncomfortable as Sirius is often seen as a protector figure for Harry.
One interesting plot development could be how Harry's relationship with the wizarding world changes. Knowing Sirius is his father might make him feel more confident and entitled to certain things. He might demand more answers from Dumbledore about his family's past and the secrets that were kept from him. Another aspect could be his relationship with the Weasleys. It might become a bit strained as he realizes his true lineage and starts to see himself in a different light, perhaps more of a pure - blood heir in his own mind, which could contrast with the Weasleys' more humble status.
One common theme is loyalty. Their friendship was always based on loyalty in the original stories, and in fan fictions, this loyalty often translates into a deep and abiding love. For example, Sirius is always loyal to Remus even when others might shun him because of his werewolf condition.
One reason could be Harry's impulsiveness. Harry often jumps into dangerous situations without thinking, and Sirius, being his godfather and having a protective nature, would shout at him to make him realize the risks. For instance, if Harry decides to go after a Death Eater alone.
Sirius could help Harry cope by sharing his own experiences of being an outcast in his family. He could tell Harry that just because his parents did this horrible thing, it doesn't mean he's unlovable. Sirius could also introduce Harry to his friends in the wizarding world, like Remus, who could also offer support and friendship.