One interesting plot twist could be that Bella realizes she doesn't want either of them and decides to go off on her own journey of self - discovery. This would completely change the dynamic of the love triangle.
Another plot twist could be that Bella is turned into a werewolf instead of a vampire (if that's not the case in the original fanfiction). This would change her relationship with both Jacob and Edward. She would have a stronger connection with Jacob as a fellow werewolf, and Edward would have to deal with her new identity which might be very different from what he expected.
Maybe Jacob and Edward end up teaming up instead of fighting over Bella. They could discover a common threat to Bella's safety that makes them put aside their differences. For example, a new vampire or werewolf clan emerges that is after Bella, and they realize they need to work together to protect her. This would be a huge shift from the typical rivalry.
One interesting plot could be that Bella discovers she has a special power that can end the fight between Edward and Jacob. For example, she can somehow make them see that their fighting is pointless and they should respect her choice instead of fighting over her.
Bella can be a great favorite too. Despite being caught between two powerful beings, she has her own strength. She's not just a damsel in distress. She has a mind of her own and makes decisions that often surprise both Jacob and Edward. Her ability to stand up to them and not be completely swayed by their powers is quite admirable.
One possible plot twist could be that an old enemy shows up at the wedding. For example, a vampire from a different coven who has a vendetta against either the Cullens or the werewolves. Another could be that Jacob has a premonition right before the wedding that something bad will happen to Bella in the future, which makes him hesitant to go through with it. Or perhaps a long - lost relative of Bella's arrives and reveals a big family secret that shakes things up.
A great plot twist could be that Jacob discovers he has a special power that can save Bella from a deadly threat, but using it would mean sacrificing his werewolf nature. Another could be that Bella, after falling for Jacob, suddenly realizes she still has some lingering feelings for Edward when he returns, creating a love triangle full of angst all over again.
One interesting Bella, Edward, and Jacob fanfiction could be about an alternate universe where Bella has to choose between Edward and Jacob based on their different abilities to save the town from a new supernatural threat. In this story, Bella might discover a power within herself that changes the dynamics of the love triangle.
There are fanfictions where Edward, Bella and Jacob are sent to an alternate reality. In this new reality, their relationships are completely different. Bella might be a powerful vampire herself, and she has to navigate her feelings for both Edward and Jacob in this new context. It's a fresh take on their love triangle.
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 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.
Another plotline might be a fight due to a misunderstanding about Bella's human friends. Edward could be overly protective and think that her human friends are putting her in danger, while Bella believes he's being too controlling. So, they have a big argument about her freedom and relationships with humans.