Well, sometimes in these fanfictions, Draco hides his pregnancy because he's in a secret relationship. For example, if he's in love with someone from a different house or a different social standing. He might be afraid that if his pregnancy is discovered, the relationship will be exposed and there will be consequences. Also, there could be a plot where he discovers some magical mystery related to his pregnancy that makes him think he has to hide it, like there's a prophecy or some dark magic involved.
In some fanfictions, Draco hides his pregnancy because he doesn't trust the medical facilities in the wizarding world to handle it properly. He may have heard some horror stories about wizarding pregnancies gone wrong. So, he tries to deal with it on his own, relying on his own knowledge of magic and potions. He might also be hiding it because he's not sure if the father of the baby will be supportive or not, and he wants to first figure out his own feelings and options before facing the father or anyone else.
Draco and Hermione is also a pairing that shows up in these types of fanfictions. Given their different backgrounds and the initial animosity between them in the original story, Draco hiding his pregnancy from Hermione can lead to a complex story. It could involve Hermione slowly finding out and having to deal with her own feelings about Draco and the situation. There's also the pairing of Draco with a random Muggle, which can add a whole new layer as he has to hide it not only from the wizarding world but also from the Muggle's family and society.
One possible reason could be fear. Maybe Hermione is afraid of how Draco will react. Given their history in the wizarding world, with their houses being rivals at Hogwarts and different beliefs, she might think he'll reject her or the baby. Another reason could be to protect the baby. There could be some threats or dangerous situations in the wizarding world that she believes Draco's knowledge of the pregnancy would put the baby at risk. For example, there could be dark forces that target the children of powerful witches and wizards, and she wants to keep the baby's existence a secret to safeguard it.
One possible plot could be that Hiccup hides his strength to protect his village in a more unexpected way. For example, he pretends to be weak while secretly training a special group of dragons to be a surprise defense force.
Well, one possible plot could be that Harry hides his power to protect his friends from dark wizards who might target them if they knew how powerful he really was. Another could be that he hides his power to secretly investigate a mystery within Hogwarts without drawing attention.
One interesting plot could be Merlin hiding his pain from Arthur because he doesn't want to seem weak. He might endure the taunts of the knights in silence while secretly using his magic to heal his internal wounds. Another could be Merlin hiding his pain over his mother's absence or the fact that he has such a huge responsibility on his shoulders with his destiny to protect Arthur and Camelot.
One popular plot is when Draco is faced with the threat of Voldemort or his Death Eaters. He might be scared of not being able to meet their expectations or of being punished for failure. For example, in some stories, he's given a dangerous task by Voldemort and the fear of not being able to complete it drives the plot.
A common plot in Draco Malfoy kidnapped fanfiction is that Draco kidnaps a fellow student to get information. He suspects that this student has knowledge about a hidden magical artifact or a secret spell that could help him gain power or status. Maybe he's fallen from grace in the Slytherin house or among the Death Eaters, and he thinks this piece of information will redeem him. Or perhaps he kidnaps someone who he believes has seen something they shouldn't have, like a Slytherin plot against the school, and he wants to keep them quiet.
One popular plot could be that they accidentally meet in the bathroom during a school event at Hogwarts. Maybe there's some sort of magical mishap that forces them to work together to fix it. For example, a spell gone wrong has flooded the bathroom and they have to combine their magical knowledge, Hermione's book - learning and Draco's pure - blood family spells, to stop the water. Another plot might involve them hiding in the bathroom from a common enemy, like a group of bullies or a dangerous creature that has entered the school. They start off bickering but gradually realize they have more in common than they thought.
A possible plot is that Draco loses his memory and finds himself in an entirely different environment. Maybe he's found by a group of outcasts or muggles who have no idea about the wizarding world. As he tries to piece together his past, he discovers new things about himself and the world around him that he had never considered before. He could also develop new relationships outside of the pure - blood wizarding society he was used to, which would change his perspective once his memory returns.
There could be a fanfiction where Harry hides his gender to escape the constant spotlight. He might change his appearance and gender identity to live a more normal life at Hogwarts, away from all the expectations. In this type of story, he could make new friends who see him as this new identity and form unique relationships.