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 common plot twist is that Edward's departure was actually a ruse to protect Bella from a greater danger. Maybe there's a powerful vampire or a group of them that he's trying to keep her away from, and leaving her seems like the only option at the time.
Often, Bella reacts with intense heartbreak. She might isolate herself from her friends and family, feeling like she can't trust anyone anymore after Edward's betrayal.
Another plot twist could be that Percy discovers he has some hidden power or heritage as a result of the betrayal. For instance, he might find out that he has a connection to an ancient and powerful force that the gods were afraid of, which is why they betrayed him. This new power then becomes a key element in his journey to either seek revenge or prove his worth.
One common plot twist could be Batman being betrayed by a close ally like Robin. Maybe Robin gets brainwashed by a powerful villain and turns against Batman. Another could be Batman's trusted butler Alfred being forced to betray him under some extreme threat. For example, a villain might hold Alfred's family hostage and make him leak Batman's secrets.
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.
A really interesting plot twist could be that Bella's pregnancy progresses at an extremely fast rate, and the baby has powers that allow it to communicate with Bella and Edward telepathically even before it's born. This causes them to reevaluate their relationship and what they thought they knew about vampires and half - vampire children.
Well, a possible plot twist is that after the car crash, Bella's blood has some strange reaction that makes her immune to vampire bites. So, Edward is both relieved and worried as this changes their relationship dynamic. Also, the car crash could lead to a time - travel situation where Bella goes back in time to prevent the crash from ever happening. Or, it could be that the car crash causes Edward to lose some of his vampire powers temporarily, and he has to rely on Bella and human methods to survive while they figure out how to get his powers back.
One common plot twist is that Bella's mermaid identity is revealed in a very unexpected way. Maybe during a swim with the Cullens, she suddenly transforms, shocking everyone. Another is that her mermaid powers are somehow connected to the Volturi's plans, adding a new layer of danger.