One interesting plot twist could be that they are betrothed due to a magical contract made by their ancestors to end an old feud between the Gryffindors and Slytherins. Hermione at first resists but then discovers Draco has a kinder side she never knew. Another could be that in the middle of their betrothal, a dark wizard tries to use their union for his own evil purposes, forcing them to work together to stop him. And perhaps, after all the chaos, they realize they actually love each other in a way they never expected.
Well, sometimes in these fanfictions, a plot twist could be that Hermione loses her memory. Draco, who was initially just going along with the betrothal for family reasons, starts to fall for her all over again as he tries to help her remember. Or, there could be a plot twist where it turns out the betrothal was a ruse by Dumbledore before he died to bring unity between the houses, but Draco and Hermione end up taking it more seriously than anyone expected.
A great plot twist in this type of fanfiction might be that Hermione discovers she has the ability to break the betrothal bond but decides not to. This could be because she has seen a change in Draco and the wizarding world around her that makes her believe in the power of their union. Also, it could be that there's a hidden prophecy related to their betrothal that neither of them knew about until some mysterious events start to unfold, and they have to figure out how to fulfill it together.
A great plot twist could be that the Death Eater she's betrothed to is actually a double agent himself. All along, Hermione thought she was the one with the upper hand in the undercover operation, but it turns out he's been playing a long - game too. This would completely change the power dynamic between them and add a whole new layer of complexity to their relationship.
One interesting plot twist could be that Draco saves Hermione not out of pure altruism but because he discovers a dark plot against the wizarding world that Hermione unknowingly holds the key to solving. So, by saving her, he's also saving himself and his family from a greater threat.
A possible plot twist could be that Hermione gets turned into a Death Eater against her will, and Draco has to help her break free from the dark magic controlling her. This would completely reverse their typical roles.
One interesting plot twist could be that when they go back in time, they find out that their families were actually friends in the past. This completely changes their perception of each other. Another could be that they accidentally change a major event in the past, which has unforeseen consequences for the present wizarding world. For example, they might prevent a certain spell from being created, which then alters the balance of power between different wizarding factions.
In some of these fanfictions, a great plot twist is when Hermione gets cursed and Draco has to find a way to break it. During this process, they learn more about each other's true selves. Also, it could be that Draco is under the influence of a dark spell that makes him act against Hermione, but she believes in the good in him and tries to break the spell, which deepens their bond in a very unexpected way.
One interesting plot twist could be when Hermione discovers her true heritage and has to face the fact that she's related to the Malfoys. It completely changes her relationship with Draco. Instead of being enemies, they have to figure out how to be siblings in a world where their families have such different values.
One interesting plot twist could be that despite their initial animosity, Hermione and Draco find they have very similar parenting styles when it comes to discipline. They both believe in strict but fair rules, which surprises everyone around them. Another could be that in the parenting class, they are paired for a project that forces them to spend a lot of time together outside of class, and during this time, they discover a mutual respect for each other's intelligence.
One popular fanfiction with this theme is 'The Green Girl' where the relationship between Hermione and Draco is explored in great detail within the context of their betrothal. It delves into how they deal with their initial differences and prejudices from their respective Hogwarts houses.
One interesting plot twist could be that Draco's family initially opposes the relationship but then comes around when they see how much he loves Hermione. Another could be that Hermione discovers a new magical ability during her pregnancy that has never been seen before. Maybe it's related to the combination of her and Draco's magic. A third plot twist might be that there is a threat to their unborn child from an old enemy of either the Malfoys or Hermione's friends, forcing them to team up with unexpected allies to protect their family.
One common plot twist is that the child has some extraordinary magical power that neither Hermione nor Draco expected. Another is that the secret of the child's existence is threatened to be exposed by an old enemy. Sometimes, the child may turn out to be the key to solving a big mystery in the wizarding world.