In some versions, Edward's payment might be a way to buy Bella's loyalty in a world where there are many factions vying for power. Bella, being a strong - willed character, might use this situation to her advantage. She could start to manipulate the situation so that she not only gets the money but also gains a position of power in the vampire - human relations, all while slowly falling in love with Edward.
One possible plot could be that Bella initially refuses because she feels insulted by the idea of being paid. But then she realizes Edward has some deeper, unspoken reasons for his offer, like protecting her from some danger in the vampire world. So she agrees and they start a journey full of secrets and unexpected twists.
Perhaps in the fanfiction, Edward pays Bella to marry him as a way to keep her close while he tries to figure out his own feelings. This could lead to a lot of tension between them, especially when Bella starts to develop real feelings for him. And as they go through the motions of a 'paid' marriage, they discover things about each other that they never knew before.
The relationship develops in a rather complex way. Since Edward pays Bella to marry him, there's an initial power imbalance. However, as they live together, they start to understand each other better. For example, Edward might learn about Bella's dreams and fears, and Bella might come to understand Edward's reasons for his actions in the vampire world. This leads to a deeper emotional bond that goes beyond the initial transaction.
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.
One possible reason could be his over - protectiveness. Edward might think that marriage would legally bind Bella to him and he could better protect her from the various dangers in their world, such as other vampires. He may believe that as his wife, he could keep a closer eye on her.