In a lot of these fanfictions, the theme of transformation is present. Bella might transform in the way she views the world and her place in it because of the Lost Boys. And the Lost Boys might also transform from being wild and carefree to having a more serious and committed side because of their love for Bella. It's really interesting to see how these themes play out in different stories.
You can try some popular fanfiction websites such as Archive of Our Own (AO3). It has a large collection of various fanfictions, and you might be able to find some great 'dwayne and bella fanfiction' there.
There could be a fanfiction where Dwayne and Bella are in a high - school setting. Dwayne is the new kid, and Bella befriends him. They then get involved in a school project that ends up changing the whole school for the better. This kind of story can explore friendship and teamwork in a relatable environment.
One possible fanfiction idea could be that Bella, as David's sister, has a conflicted relationship with the vampire gang. She might try to keep David from getting into too much trouble while also being drawn to their wild lifestyle. For example, she could be the voice of reason when they plan their hunts, but at the same time, she has her own vampire instincts that she struggles with.
One popular story could be 'Bella's Encounter with the Lost Boys'. In this fic, Bella stumbles upon the Lost Boys' hideout and a whole new adventure begins. There's a lot of character interaction and the author does a great job of building tension between Bella and the boys.
One interesting story could be where the Lost Boys meet Bella Swan in a different dimension. They initially see her as a strange outsider but gradually come to respect her tenacity. Maybe she has some unique power that helps them in their vampire world.
There's a fanfiction called 'Bella and the Lost Boys: A Forbidden Love'. This one really focuses on the idea of their love being something that goes against the norm. It shows how Bella and the Lost Boys struggle with their feelings and the consequences that might come with their relationship. The descriptions of their interactions are quite vivid, making it a great read.
There are also some smaller, more niche fanfiction communities dedicated to specific fandoms or crossovers. You can try to find ones related to either 'The Lost Boys' or 'Twilight' and then look for the crossover stories within those communities. Social media groups might also be a good source. Some groups allow members to share and discuss their favorite fanfictions, and you might get lucky and find some great 'lost boys x bella swan' stories there.
The story could also take a different turn in terms of the plot. If Bella is David's sister, she might have some influence over the group's decisions. For instance, she could convince them to target different prey or to avoid certain areas. This would change the way they interact with the human population in Santa Carla. And if there are other vampires who are enemies of David's gang, they might use Bella as a way to get to David, adding a whole new layer of conflict.
First, know the characters well. Read the original works about Bella and understand the nature of the Lost Boys. Then, come up with an interesting plot, like a mystery they have to solve together.