One possible plot twist could be that Bella discovers Edward has a secret identity that is not what it seems, like he's part of a supernatural group she didn't know about. This could change her perception of the forced marriage and add a whole new layer of drama.
A plot twist could be that Edward himself doesn't want to marry Bella in the way it's being forced. He might have his own reasons, like he's in love with someone else but is bound by family honor or some other obligation. This could create a love triangle - like situation within the forced - marriage scenario.
Maybe Bella finds out that her parents were forced to make her marry Edward by some external force, like a powerful family or a magical curse. She then has to decide whether to fight against this external force or try to understand it, which could lead to some interesting character development.
She could try to reason with her parents. She could tell them that she doesn't love Edward and that forcing her into this marriage will only make everyone unhappy. She could also point out the modern values of freedom in choosing a life partner.
In this kind of fanfiction, a really interesting plot twist could be that the princess and the prince are from two warring kingdoms. The forced marriage is seen as a way to end the war. But as they get closer, they find out that there are other forces at work trying to keep the war going. So, they have to not only deal with the forced aspect of their relationship but also figure out who is behind the plot to keep their kingdoms at war. This could involve them going on a secret mission together and falling in love in the process, which completely changes the nature of the forced marriage.
One possible reason could be his over - protectiveness. Edward might think that marriage would legally bind Bella to him and he could better protect her from the various dangers in their world, such as other vampires. He may believe that as his wife, he could keep a closer eye on her.
One possible plot development could be Bella trying to find a way to escape the forced marriage. Maybe she discovers some hidden secrets about Edward or the circumstances that led to the forced union, which gives her leverage to break free.
Another plot twist could be that Bella herself realizes she has made a mistake in choosing Edward over her'real mate'. She might start having dreams or visions of a different future with her real mate. This could lead to a lot of internal conflict for her as she has to decide whether to stay with Edward or go back to the one she was meant to be with.
Another possibility is that Edward betrays Bella by revealing her secrets to their enemies. Maybe he was forced to do so under some sort of blackmail from a powerful and malicious vampire or other supernatural being. In this case, Bella would have to face the consequences of having her secrets exposed while also dealing with Edward's betrayal.
One common plot twist is that Bella might be in danger not just from the typical vampire threats but from a human organization that knows about vampires. Edward then has to save her while also hiding the existence of vampires from this group. For example, in some fanfics, there are hunters who target Bella to get to Edward, and Edward has to outwit them.
One possible plot twist could be that Bella discovers Caius has a hidden agenda all along, and it's not just about their relationship. Maybe he's using their bond to gain power within the vampire world. Another twist might be that Bella has a secret power that is awakened by her connection with Caius, and this power changes the dynamics of their relationship and the entire vampire community.
One interesting plot twist could be that the treaty has a hidden clause which changes the nature of their marriage completely. For example, it might initially seem like a simple political union, but the hidden clause reveals that they have to perform certain magical or supernatural tasks together to truly solidify the marriage.
One possible plot twist could be that Edward realizes that Bella represents a more 'normal' life for him compared to Fem Harry. Maybe Fem Harry has some dangerous or complex magical powers that Edward fears, while Bella seems more human and easy to understand.