In some fanfictions, a plot twist could be that Jacob and Bella are not actually the ones getting married at the end. There could be some magic or spell that makes them swap bodies with other characters who then end up at the altar. Another idea is that a powerful witch shows up and curses the wedding, saying that if they go through with it, all the werewolves will lose their powers and all the vampires will be weakened. This would create a huge dilemma for Jacob and Bella as they have to decide between their love and the well - being of their respective clans.
Well, it might be that there's a mix - up with the wedding rings. Maybe they get lost or switched with some other rings that have a special meaning in the vampire - werewolf lore. Also, there could be a sudden change in the weather that is not natural but caused by some supernatural force that doesn't want the wedding to happen. And what if Bella discovers a new power right at the moment of the wedding that she has to deal with immediately.
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.