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.
It could be a werewolf. In the Twilight universe, werewolves and vampires often have a complex relationship with humans. A werewolf might have been Bella's real mate, perhaps one that she met briefly before Edward entered the picture. Maybe this werewolf had a special connection with Bella that was disrupted when Edward came along.
Another reason could be a different interpretation of the characters' personalities. The writer might think that in the original, the idea of them being mates was too forced. So, by making Bella not Edward's mate, they can develop the characters in a way that they believe is more true to their individual natures, perhaps showing that Bella has different values or desires that don't align with being Edward's mate.
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.
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.
One possible plot twist could be that Edward's family doesn't approve of Bella's job at the stripper club. This could create a lot of tension between them and force Bella to make a difficult choice between her relationship and her job.
There could also be an external threat that forces them to put aside their differences. For example, a new vampire or a group of enemies might attack their community. In this situation, Edward, Bella, and Jasper have to work together. During this time, Edward might realize that his jealousy is unfounded, and they all form a stronger bond as a result.
She might be in shock at first. After all, she was in a life - threatening situation. But then she could feel extremely grateful towards Edward and their relationship might grow stronger.
One possible reason could be that Edward believes Charlie is a threat to Bella in some way that Charlie doesn't realize himself. Maybe Charlie unknowingly gets involved with some dangerous elements, and Edward, with his vampire senses, detects it.
In some fanfictions, they could face new threats from other supernatural beings. Maybe a new coven arrives in town, and they have to protect their family and the town. This would put their relationship to the test as they deal with the danger together.
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.