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.
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.
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.
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.
In the fanfiction, the development could be that Bella begins to have doubts about Edward's ability to fully be part of her human world. Meanwhile, Jacob is always there for her, sharing his own dreams and fears. Their relationship deepens gradually, and Bella starts to see a future with Jacob that is more stable and less complicated than the one with Edward. So, she makes the choice to be with Jacob.
One reason could be Jacob's more down - to - earth nature. He represents a more normal, warm - hearted life compared to Edward's rather cold and sometimes brooding vampire existence. Bella might be drawn to the simplicity and closeness to nature that Jacob offers.
I think Stefan can be a really liked character in these fanfictions even though he's in a bit of a love triangle situation. His complex relationship with Damon and his past with Elena make him an interesting addition. He might be shown as still caring for Elena but also coming to terms with her love for Damon. Also, Alaric is a character that many fans like. He can act as a mentor or a voice of reason in the fanfiction, and his relationship with Damon and Elena can be explored in different ways.