In some fanfictions, the arranged marriage might be a part of a bigger plan to unite their families against a common enemy. So, Edward and Bella have to put aside their personal feelings and work together. This could lead to some really intense and emotional moments between them as they learn to trust and rely on each other.
A possible plot is that Bella has to adapt to the vampire world quickly because of the arranged marriage. Edward has to teach her about their ways and in the process, they form a deep bond. There could be cultural clashes as well, like Bella's human values conflicting with the vampire traditions in Edward's family, which makes for a very engaging read.
One interesting plot could be their exploration of a new, mysterious island during their honeymoon. They might discover hidden caves with ancient secrets.
One interesting plot could be where Edward and Bella face a new supernatural threat together that tests their relationship in ways not shown in the original story. For example, a new type of vampire with unique powers arrives in Forks.
Another plot might involve an external threat to the royal family that Edward and Bella have to face together. Maybe a neighboring kingdom is planning an attack or there is a magical threat that only their combined powers (if they have any in the fanfiction) can overcome. This would test their relationship as they fight for the safety of their kingdom and each other.
Another plot could be that there's some sort of mystery or danger at the prom. Maybe a rival vampire group shows up, and Edward has to protect Bella while still trying to make their prom night special. They might have to dance and be romantic in between fending off attacks.
One interesting plot could be the child having special powers inherited from both Edward and Bella. For example, the child might have the ability to read minds like Edward and also possess Bella's strong will. Another plot could be the child's struggle to fit in with both the vampire and human worlds. Maybe they face challenges at school because they are different from the other kids.
One interesting plot could be where Edward and Bella face a new kind of vampire threat together. They have to use their unique bond as mates to overcome it. For example, a group of vampires with special powers that can disrupt their mental connection.
One possible plot could be that Edward returns to Bella after a long self - discovery journey. He realizes that his love for her is unwavering. Another might be that he comes back to protect Bella from a new and dangerous threat in Forks.
One possible plot could be that a group of rogue vampires, jealous of Edward and Bella's relationship, kidnap them. The kidnappers might try to use Bella to get to Edward's family secrets. Another plot could involve a powerful witch who wants to use their unique vampire - human bond for some dark magic purpose. Maybe they are held in an abandoned castle, and they have to rely on their love and unique abilities to escape.
In some fanfictions, their marriage might lead to the discovery of new powers. For example, Bella's unique human - turned - vampire nature combined with Edward's abilities could result in a special power that helps them protect their family. It could be something like a telepathic bond that is even more powerful than Edward's existing one.
In some Edward and Bella marriage fanfiction, there could be a plotline where they face threats from other vampire clans who are not happy about their union. This would lead to a lot of action and drama, with Edward and Bella having to fight to protect their marriage and their family. They might have to rely on their friends and allies within the vampire and human worlds to overcome these challenges.