One possible plot twist could be that it was all a dream or a vision caused by a powerful magical artifact. Harry wakes up thinking it's real, and then he has to figure out what the vision means. Maybe it's a warning about some future danger related to Severus, and the pregnancy is just a symbolic representation of something they need to protect or create together in the future.
Perhaps there's a time - travel element. Harry accidentally travels back in time and does something that causes Severus to be in this strange 'pregnant' state in the future. They then have to work together to reverse the effects while also dealing with the changes it has brought to their relationship. Along the way, they learn about each other's pasts more deeply and form an unexpected bond that helps them in their quest to set things right.
A plot twist could be that a mischievous magical creature is playing tricks on them. It makes Severus appear pregnant to Harry as part of its prank. But as they investigate, they discover that the creature is actually trying to lead them to a hidden magical secret or an ancient spell that could be used to save the wizarding world. So, the 'pregnancy' becomes a starting point for a grand adventure.
One possible plot twist could be that the pregnancy is the result of a magical accident or spell gone wrong. Maybe they were working on a new charm and something unexpected happened. Then they have to figure out how to deal with it while keeping it a secret from their enemies in the wizarding world.
One possible plot twist could be that the girl who is pregnant has some sort of magical ability related to the pregnancy that no one expected. Maybe she can communicate with the unborn child in a magical way. Another could be that Harry discovers he's not the real father, but due to some magical mix - up, he has to take on the role anyway.
A really interesting plot twist could be that after getting pregnant, Lily loses her memory. So Snape has to deal with not only the fact that he's going to be a father but also that Lily doesn't remember their relationship. And then James steps in to help, thinking the baby is his. This would create a very strange love triangle situation. Also, what if the baby Harry has some sort of special power from the start that no one expected, which could be a major plot twist.
One possible plot twist could be a magical mix - up. Maybe a spell gone wrong during a joint mission between Harry and Fleur. For example, a spell that was meant to protect them instead created a false pregnancy - like situation for Fleur. Another could be a prophecy that was misinterpreted, making everyone think Fleur was pregnant when in fact it was a sign of something else entirely related to the magic in the wizarding world.
It could be a story where Luna has a premonition about having a child with Harry. They initially don't believe it but as time passes, some strange magical signs start to occur that seem to confirm it. So they start preparing for the baby in a very sweet and naive way. They might ask for advice from Hagrid on how to take care of magical creatures (thinking the baby will be like a magical creature in some ways) and from Hermione on how to study up on child - rearing in the wizarding world.
Well, if we were to consider a non - inappropriate version, a possible plot twist could be a time - travel scenario. Maybe Naruto travels back in time and, instead of a romantic or sexual relationship, he ends up being in a situation where he has to protect Mikoto from some great danger. And through some magical or ninja - like means, something happens that gives the false appearance of pregnancy as part of a plot by an enemy to create chaos. So Naruto has to clear his name and also save Mikoto from the real threat.
One possible plot twist could be that Severus initially denies the child is his, leading to a rift between him and Harry. Then, as the pregnancy progresses, he realizes his mistake and has to fight to win Harry back. Another could be that the wizarding world discovers the pregnancy and there is a huge outcry against it due to their past relationship, forcing Harry and Severus to go into hiding.
A plot twist could be that Karin's pregnancy is a false alarm, but in the process of believing it was real, Naruto and Karin realize their true feelings for each other. They might have been in denial before, but this situation forces them to confront their emotions and they end up starting a real relationship.
One possible plot twist could be that Lily discovers she has some sort of magical complication during the pregnancy that endangers her life or the baby's. This would add a lot of drama and tension to the story.
One possible plot twist could be that the pregnancy is part of a larger scheme by a third party. Maybe Voldemort or some other dark wizard has manipulated the situation to either gain control over Lucius or to use Harry's unborn child for some dark purpose.