Perhaps the captors have some ulterior motives for restraining the girl. It could be that she is a key to something they want, like a special power or a valuable secret. As the story unfolds, she might start to understand their reasons and use that knowledge to her advantage. For example, she could pretend to cooperate in order to get closer to the source of their power and then turn the tables on them.
One possible plot development could be that the girl manages to break free from the captors and then sets out to expose their wrongdoings. Maybe she has some hidden strength or knowledge that helps her in this escape.
You can add a lot of mystery. For example, the captors could be wearing masks and not revealing their identities or motives at first. This will keep the readers guessing and make them more interested.
One possible plot development could be the immediate rush to get medical help. Maybe some bystanders call an ambulance and the girl is rushed to the hospital where the doctors fight to save her life.
One possible plot development could be that Harry has to face the challenges of being a young father while still dealing with his wizarding duties. He might struggle to keep his family safe from the dark forces that are always lurking in the wizarding world.
Another development could be that it causes some conflict within their group of friends. Some might be accepting right away, while others might be skeptical or have concerns about how this will affect their team dynamics. There could be a lot of discussions and arguments about whether Naruto should continue to be part of the group in the same way.
One possible theme could be the struggle for survival. The girl might be in a situation where she has to fight against the drugging effects and her captors to stay alive. Another theme could be rescue, perhaps there will be someone coming to save her from this harrowing situation.
One possible plot development could be that Harry discovers new magical abilities or knowledge within the Greengrass family. Maybe they have a unique magical heritage that helps him in his fight against Voldemort.
One possible plot development could be that Naruto discovers his deafness gradually and has to hide it from his enemies. He might be afraid that they would use it against him in battle. Another could be that his friends find out by accident and have to learn sign language to communicate better with him.
One possible plot development could be Percy having to face various ancient and terrifying monsters on his own in Tartarus. He might have to use his wits and limited resources to survive. For example, he could encounter the monstrous Titans that were imprisoned there long ago.
In this type of fanfiction, it could be that the very essence of Tartarus affects the pregnancy in strange ways. Maybe the unborn child starts to show signs of being influenced by the dark magic or the primordial forces there. This could lead to a race against time as they try to find a way to counteract these effects. Also, the relationship between Annabeth and Percy would likely be put to the ultimate test. They would have to rely on each other more than ever, and their love for each other and the baby would be what drives them forward. Their friends on the outside might also sense something is wrong through some sort of magical connection and start their own quest to try and reach them in Tartarus.
In this fanfiction, perhaps Harry is kidnapped by the Order because they think he's being influenced by dark forces. As the story unfolds, he could start to understand their reasons while also uncovering a traitor within the Order who wants to harm him for real. Or, he might use his skills to turn the situation around and make the Order see that they were wrong to kidnap him in the first place. He could then lead them in a new direction against Voldemort.