One possible reason could be that Bella has a certain allure or mystery that Damon finds more captivating than Elena. Maybe Bella represents a new and different kind of challenge for him.
Perhaps Damon sees in Bella a different set of values or a more unwavering loyalty. Elena has her own complex relationships and loyalties in the 'Vampire Diaries' world, and Damon might feel that Bella can offer him something purer in terms of a connection. For example, Bella's love for Edward in the 'Twilight' series is extremely intense and unwavering, and Damon could be drawn to that kind of devotion.
One possible reason could be that Bonnie has a unique strength and independence. She's a powerful witch, and Damon might be attracted to that power. Also, Bonnie has always been there for him in tough situations, more so than Elena in some ways.
This change would also have an impact on the other characters. Stefan, for example, might be caught in the middle. He has his own relationship with Damon and also cares about Elena. The fanfiction could create a whole new set of alliances and rivalries. Characters from the 'Twilight' series might also be involved in some way, further complicating the relationships. Overall, it would be a big upheaval in the existing relationship dynamics.
Elena might be heartbroken. She probably expected Damon to always be there for her, so his choice of Bonnie would come as a shock.
Elena might show her jealousy by being overly critical of Bella. For example, she could constantly find fault in Bella's actions or decisions in the fanfiction.
In this fanfiction, Bella is likely the main focus. Because the whole premise is about Elena being jealous of Bella, which implies that Bella is at the center of whatever relationship or situation is going on with Damon. Elena's jealousy is directed towards Bella, so the story would probably revolve around Bella and how she deals with Elena's jealousy and her relationship with Damon.
Damon could be a vampire or some other powerful supernatural being. Bella might be a human, perhaps with some special quality that makes her a target for James. Maybe she has a unique scent or power that she's not even aware of yet.
Caroline could be in the story. She might be Bella's friend or a rival. If she's a friend, she could help Bella navigate the supernatural world. If a rival, it could be because of her relationship with Damon or Stefan in some way. Also, Klaus could be a character. He might see Bella as a means to get to Damon, or he could be interested in her unique abilities if she has any.
There could be Bella's mother. She would be an important character as she was part of the relationship that led to Bella's birth. Maybe she has some secrets or reasons for keeping Damon away from Bella all these years.
Their relationship becomes more serious and committed. With a baby on the way, they have to think about more than just themselves.
One aspect of their married life in fanfiction could be their social life. They might host parties for their friends, both human and supernatural. Elena would be the warm hostess, and Damon would be the charming one, entertaining the guests with his witty remarks. And at night, they would just enjoy the quiet togetherness, maybe stargazing and reminiscing about their long and eventful journey to get married.