A great plot twist could be that Bella's amnesia makes her develop new powers. Since she can't remember her past, her mind is in a different state, and this could unlock something within her. Maybe she can now see the future or has some kind of mind - control ability. Also, it could be that the amnesia was a ruse by the Volturi to test Edward and Bella's relationship. They could be watching from afar to see how Edward reacts to Bella not remembering him.
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.
Well, in some Bella has amnesia fanfictions, a plot twist might be that her amnesia is a result of a spell gone wrong by a witch in the Twilight universe. This could lead to her having strange powers or abilities that she didn't have before. Also, it could be that her amnesia causes her to forget not only her personal memories but also all knowledge of the vampire and werewolf existence. So when she starts to regain her memories, it's a whole new discovery for her again.
One possible development could be that the relationship between Bella and Edward becomes even more complex. Edward might feel a stronger sense of protectiveness towards her. Also, it could lead to a rift between Bella and Jacob, as Jacob may not be able to handle the situation well. The Cullen family would also be greatly affected, having to deal with a pregnant human in their midst.
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.
Well, in this kind of fanfiction, a plot twist could be that an outside force tries to drive them apart by making them believe false things about each other. Maybe they are both in love with the same person, which creates a huge rift between them. Or there could be a situation where one of them sacrifices themselves for the other in a way that no one expected, completely changing the dynamic of their relationship.
One possible plot twist could be that Paul is actually working for an enemy of Bella's family in secret, and his initial meeting with Bella was part of a plan. But then he starts to have real feelings for her and has to decide between his loyalty to his side and his new - found feelings.
Another plot twist could be that Aro has some grand plan for Bella that goes against what she wants. For example, he might want her to marry a certain vampire from his coven to strengthen their family's power, but Bella is in love with Edward. This would create a huge conflict between her loyalty to her new - found family and her love for Edward and the Cullens.
One possible plot twist could be that Jasper is being controlled by an external force, like a powerful vampire with mind - control abilities. So, he doesn't really mean to kidnap Bella in his true self.
One possible plot twist could be that Jasper discovers some hidden power or heritage in Bella that he wasn't aware of before. This could change how he views her and their relationship. Another twist might be that there's some sort of enemy from Jasper's past who targets Bella because of her relation to him. It could also be that Bella finds out some dark secret about Jasper that makes her question whether she can really trust him as a father.
Maybe Victoria has actually set a trap for Jasper while pretending to target Bella. So when Jasper goes to save Bella, he walks right into the trap. Or perhaps there's a misunderstanding and Bella thinks Jasper is working with Victoria at first, which adds a whole new layer of complexity to the story.