Well, it could be a very strange and complex plot. Maybe there's some sort of magic gone wrong that allows for this impossible situation. Sirius might initially be shocked and then have to deal with the consequences, like helping Severus through the pregnancy while also facing the judgment of their wizarding community.
In such a fanfiction, the plot could involve a magical experiment that backfires. Sirius and Severus, who are often at odds in the original story, are now forced to cooperate. Severus, being pregnant, has to deal with the physical and emotional changes, and Sirius has to take on the role of a supporter. There could also be a sub - plot about how they keep this a secret from their friends and enemies alike, and how it ultimately changes their relationship from one of enmity to something more complex and intertwined.
I'm not sure if there is such a specific fanfiction. Fanfiction can cover a wide range of themes and ideas, but this one seems rather unusual. You could try searching on popular fanfiction websites like Archive of Our Own or FanFiction.net to see if it exists.
There may be such fanfiction in some niche corners of the internet, but this goes against the original nature of the Harry Potter series and is also a rather strange and inappropriate concept. J.K. Rowling's original works don't have any such plotlines, and most official and mainstream interpretations stay true to the original characters' relationships and story arcs.
There might be some out there. Fanfiction writers can come up with all sorts of creative and sometimes out - of - the - ordinary ideas. However, this one seems quite unique and might not be as widespread as more common fanfiction themes in the Harry Potter universe. Some smaller, more niche fanfiction communities might be a place to look, but again, it's hard to say for sure.
A possible plotline could be centered around Severus seeking help from unexpected sources. Maybe he turns to some of the more neutral or even good - aligned wizards and witches for advice or assistance, since he can't rely on Voldemort or the Death Eaters completely. The pregnancy could also be a catalyst for Severus to re - evaluate his life and his actions. He might start to question his past decisions and try to find a way to create a better future for himself and the unborn child, all while dealing with the stigma and danger associated with such a situation in the wizarding world which is full of secrets and power struggles.
It could be a new character created specifically for the fanfiction. Maybe a witch who Sirius meets during one of his adventures away from his usual group of friends.
One major problem is that it goes against the nature of the relationship between Sirius and Harry in the original 'Harry Potter' series. Their relationship is more of a mentor - mentee or a father - son like bond, not a romantic or sexual one.
A possible plot could be that they are both trapped in a magical dimension. They have to rely on each other to find a way out. During this time, they share their stories and secrets. Snape reveals his true feelings about Lily, and Sirius tells about his family. This mutual sharing helps them see each other in a new light and forms the basis of their friendship.
This is a rather strange and non - canon concept. However, in such a fanfiction, it might explore a very different magical world where male pregnancy is possible. It could involve a deep emotional bond between Sirius and Harry that goes beyond the normal bounds of their relationship in the original series. There would likely be a lot of drama and angst as they deal with the unexpected situation and the reactions of their friends and the wizarding community at large.
It's a very niche and rather unusual concept, so it's not likely to be a common type of fanfiction. However, there may be some out there in the far reaches of the fanfiction world. But it might be difficult to find one that is well - written and also adheres to the overall tone and feel of the Harry Potter universe while still exploring this very unique idea.
One possible storyline could be that Sirius initially panics when he finds out about the pregnancy. He might struggle with his responsibilities as he has a rather wild and carefree nature in the original story context. He could then gradually grow into the role of a father - to - be, facing challenges from his friends and family who may have different expectations.