One possible scenario could be that Bella meets a new vampire who has a very different charm from Edward. Maybe this new vampire is more carefree and shows Bella a different side of the vampire world that she becomes attracted to, leading her to cheat in the fanfiction.
In some fanfictions, there could be a dark magic or spell involved. A witch or a dark force might manipulate Bella's feelings, making her believe she has fallen for someone else, and thus she cheats on Edward. This could add an element of mystery and the supernatural to the story beyond just the vampire - human love story we know from the original Twilight series.
Perhaps in the fanfiction, Bella feels neglected by Edward. He might be away on some important vampire business for a long time. During this time, she grows close to a human friend who understands her in a different way, and she ends up cheating on Edward with this friend. However, this would be a very different take on the Twilight world as Bella's love for Edward was supposed to be very strong.
One possible scenario could be that Edward sacrifices himself to save Bella from a powerful new vampire threat. Another might be that he succumbs to an illness that vampires in this fanfic world are suddenly vulnerable to. Or perhaps he is killed in a battle against a group of werewolves gone rogue.
One possible consequence could be Bella's heartbreak. She would likely feel betrayed and lose trust in Edward. Another consequence might be a rift in the vampire community if the affair becomes known. Tanya could also face backlash from other vampires who support Bella and Edward's relationship.
For Bella, a nightmare could involve her not being able to control her vampire instincts and accidentally harming someone she loves, like her father or one of her human friends. This would be a huge source of distress for her. Carlisle might have a nightmare where the Cullen family is ostracized by the vampire community for some reason, like breaking an important vampire law. He would be devastated as he has worked so hard to build a harmonious existence for his family within the vampire world.
Perhaps in the fanfic, they have encounters with local supernatural beings that are unique to the honeymoon location. This could add an element of mystery and adventure to their time together. For example, they might meet a friendly spirit that gives them advice about their future as a couple.
Well, there are likely to be some 'Bella cheats with Edward' Twilight fanfics in the vast world of fanfiction. Fanfic writers like to play with different ideas and scenarios. But this particular scenario is quite a departure from the original story's portrayal of their relationship. It might be interesting to read for those who enjoy exploring different relationship dynamics in fan - created content, but it may also shock or disappoint traditional Twilight fans.
One possible scenario could be that Bella is initially terrified but then realizes there's a deeper connection to Edward's bite. Maybe it unlocks some sort of hidden power within her. Another could be that Edward immediately regrets his action and tries to reverse the effects. It could also be that the bite sets off a chain of events in the vampire community, as it's an unexpected and early bite.
In certain 'Bella dies in Edward's arms' fanfictions, it might be related to time - travel elements. Imagine a situation where Bella travels back in time to change something for the better in Edward's past, but in doing so, she disrupts the timeline in a way that causes her own death. When she returns to the present (or what was the present before the time - travel), she is already dying and Edward holds her as she passes away. Another idea could be that Bella's human frailty, combined with the dangerous vampire world she's a part of, leads to her being in a situation where she can't be saved, and Edward has to watch her die in his arms.
There are other fans who are interested in exploring different storylines. They see it as a creative way to expand the Twilight universe. They might enjoy seeing how the characters would react in such a difficult situation, even if it's not in line with the original story.
A possible plot might involve Rosalie having some sort of change in her feelings towards Edward. She might start to show him more affection than usual. Edward, being a bit confused by his own emotions, succumbs to her advances while still being in a relationship with Bella. This could lead to a lot of drama, with Bella finding out and feeling heartbroken.
Many fans would likely be divided. There would be those who are open to new interpretations and might find it an interesting take on the characters. However, a large portion would probably be against it as it goes against the established love story between Bella and Edward. They might feel that it changes the essence of the characters as they were presented in the original books.