Maybe there's some sort of plot or secret that he doesn't want Hermione to expose. He might think that by keeping her close, he can prevent her from telling others. For example, if he's involved in some sort of illegal or unethical magic practice and he believes Hermione has figured it out.
Hermione could leave Draco if she realizes that they have fundamental differences in values. For example, if Draco still holds some pure - blood supremacist views, Hermione, being a Muggle - born advocate, would not be able to tolerate it in the long run.
One popular theme could be the idea of redemption for Draco. In many fanfictions, he is shown to overcome his family's dark past and prejudice. Another theme might be the power of friendship or love that develops between Hermione and Draco, despite their initial differences. It could also explore how they navigate the complex wizarding world together, facing challenges from their respective houses and the general wizarding society.
It's also possible that other characters would get involved. Harry and Ron, being Hermione's closest friends, would surely notice her absence. They could come to her rescue. There might be an epic battle of magic between them and Draco, or they could try to reason with Draco, appealing to his better nature if he has one.
She could also start to negotiate with Draco. Given her intelligence, Hermione might try to understand Draco's motives and then use reason to persuade him to let her go. She might offer alternatives or compromises that would satisfy both of their needs.
In a fanfiction with this theme, it could be set post - war. Draco and Hermione find themselves in a situation where they are on the run from some dark wizards who are after them for different reasons. During their escape, Hermione gets wet and cold. Draco, having a change of heart after the war, gives her his warm cloak. This act of kindness begins to break down the barriers between them, and they start to have deeper conversations, realizing they have more in common than they thought.
Perhaps there's a fanfiction where a powerful and dark wizard from outside the school is targeting Muggle - borns. Hermione, being a Muggle - born, is understandably afraid. Draco, who has always had a complex relationship with Hermione, is conflicted. He doesn't want to see her hurt because, deep down, he has feelings for her. He ends up using his Slytherin cunning to outwit the dark wizard and keep Hermione safe.
One reason could be the traditional rivalry from the original series. In the books, they are from different houses, Slytherin and Gryffindor, which have a long - standing feud. Fanfiction writers might play on this by having Draco be mean as a continuation of that in - built tension. Another aspect could be his family upbringing. The Malfoy family has certain pure - blood supremacist views, and Hermione being a Muggle - born might be a target for Draco's meanness in fanfictions.
One alternative theme could be a time - travel story where Hermione and Draco are sent back in time and have to work together to preserve the wizarding world's history. It's a fun way to explore their characters in a new setting.
She could stand up to him. Hermione is known for her intelligence and quick - wittedness. So she might use her sharp tongue and logical arguments to put Draco in his place. For example, she could expose his wrongdoings or the flaws in his pure - blood ideology.
In a different fanfiction, Hermione could be afraid of a prophecy that involves her and Draco. The prophecy seems to foretell doom and destruction. She's terrified of what might come to pass. Draco, who also hears about the prophecy, tries to comfort her. He tells her that prophecies can be misinterpreted and that they can change their fate together. He starts to work with Hermione to figure out a way to avoid the bad outcomes predicted by the prophecy.