A really unexpected plot twist could be that the baby isn't actually Sebastian's. There could be some sort of magic or deception at play that makes everyone believe it's his, but in reality, it's someone else's, which would add a whole new layer of drama and mystery to the fanfiction.
One possible plot twist could be that Sebastian's true motives for getting Clary pregnant are revealed to be part of a larger, darker scheme. Maybe he plans to use the baby for some sort of power - play within the Shadowhunter world.
On the other hand, there could be a glimmer of hope in their relationship. Maybe through the shared experience of the pregnancy, they start to see the good in each other. Clary might see a side of Sebastian that she never knew existed, and Sebastian might start to have real feelings for Clary and the baby, leading to a more complex and evolving relationship.
One possible plot could be a twist where Sebastian has some sort of redemption arc. Maybe he discovers that Clary is pregnant and realizes he wants to change for the sake of the unborn child. He starts to fight against his dark side and tries to protect Clary and the baby from the threats in their world.
A possible plot twist in a Regina Gets Pregnant Fanfiction could be that the pregnancy progresses at an extremely rapid rate. Instead of the normal nine months, it might be just a few weeks. This would force Regina to quickly adapt to the idea of being a mother and also deal with the physical and emotional changes in a very short time. Additionally, what if the baby is a key to unlocking a long - forgotten magic in the realm? This would make Regina a target for those who want to use the baby's power for their own gain. Or, it could be that Regina's pregnancy is a sign of a new era in the magical world, and she has to figure out how to lead the way into this new future while carrying the baby.
One plot twist could be that the 'pregnancy' actually turns out to be a way for Kyuubi to transfer its remaining power to Naruto in a more permanent and extreme way. Instead of just lending power, it's like a total fusion of their essences. Another could be that the 'pregnancy' is a ruse by an enemy who has manipulated Naruto's connection with Kyuubi to make it seem like this is happening, when in fact they are trying to steal the power that would be created from such a union.
A cool plot twist could be that the pregnancy is a sign of Danny's evolution into a new form. Instead of it being a normal biological pregnancy, it's a supernatural transformation. As he progresses through the 'pregnancy', he gains new powers and abilities that he has to learn to control, and in the end, he emerges as a more powerful version of himself.
The theme of sacrifice is also quite common. They might have to sacrifice their own desires, like their freedom or their previous life goals, for the sake of the baby. Also, the theme of identity. With the pregnancy, they both have to re - evaluate who they are. Clary is no longer just a young shadowhunter, but a soon - to - be mother. Sebastian has to figure out if he can be a father and what kind of father he wants to be, considering his dark past.
One interesting plot twist could be that Sherlock initially doesn't believe he's the father and tries to use his deductive skills to disprove it, only to find out he was wrong all along. Another could be that Molly tries to keep the pregnancy a secret from Sherlock at first, but then circumstances force her to reveal it in a very dramatic way, like during a dangerous case. And sometimes, there might be a plot twist where other characters, like Mycroft, try to interfere with the situation for their own reasons, either to protect Sherlock or because of some secret agenda they have regarding the potential child.
A third plot twist could be that the attack is a diversion. While Clary is fighting the demon, something much more significant is happening elsewhere. Maybe a powerful artifact is being stolen or a key location is being infiltrated, and by the time she realizes it, she has to race against time to stop the real danger.
Maybe the person Amy thought was the father isn't actually the father. This revelation could create a lot of drama and conflict in the fanfiction. It would lead to her having to figure out who the real father is and how to deal with the two men involved.
A possible plot twist could be that Astrid discovers she has some special powers during her pregnancy that are related to the dragons. This could change the way they view the pregnancy and the future of their child. Another twist could be that Hiccup is told he has to go on a very dangerous mission right when Astrid is in a critical stage of pregnancy.