Link is obviously a popular choice. His journey, his skills, and his relationships can be explored in many different ways. Midna is also very popular. Her mysterious nature and her connection to the Twilight Realm make her an interesting character to write about.
Jacob Black is also quite popular. His loyalty and his struggle between his love for Bella and his werewolf identity can add a lot of depth to a fanfiction. Another popular character could be Alice Cullen. Her ability to see the future can create interesting plot twists and turns in a story 'into the night' in the 'Twilight' world.
Harry Potter himself could be a central character. His courage and ability to face dark forces could be put to the test in the Twilight universe. Maybe he has to deal with a vampire coven that has some dark intentions similar to Voldemort.
Obviously, Bella, Edward, and Jacob are the main ones. Bella is at the center of the love triangle, so she's always a key focus. Edward with his brooding vampire allure and Jacob with his warm werewolf charm are also very popular to write about.
Renesmee is definitely a popular one. Since she is a new and unique character in the Twilight universe, there are a lot of possibilities to explore with her. Jacob is also popular as his relationship with Renesmee is quite complex. And of course, the Cullen family members like Edward and Bella still hold a lot of interest as they are central to the whole story.
The main hero, of course. They are often the center of the action in the original story, so there's a lot of material to work with. You can explore their uncharted emotions or put them in new and challenging situations.
Edward, even though he is absent for a large part of 'New Moon', still has a big impact on the story. Fanfic might explore his time away, what he was thinking and feeling, and how his return would be different if the story was told from a different angle.
In these fanfictions, Carlisle Cullen can be a popular character. As a doctor in the Twilight series, in the billionaire fanfiction world, he could be a highly respected and wealthy doctor who is part of the Cullen billionaire family. Alice Cullen is also likely to be a popular choice. Her ability to see the future can add an interesting twist to the billionaire plot, like she can foresee business deals or threats to the family's fortune. Then there's Rosalie Hale. She might be the glamorous and sometimes haughty member of the billionaire vampire family.
Clear Rivers is a great character to focus on. Her experience in the first movie and her determination to survive make her an interesting subject for fanfiction. You could explore her life after the events of the movie and how she continues to deal with the fear of death.
Simba is a very popular choice. His journey from a cub to a king offers a lot of room for creativity in fanfiction. Nala is also great as she is strong - willed and can be the center of stories about female empowerment in the Pride Lands. Scar is another one, as his villainous nature can be explored further, like what made him so evil.
Shiro is also a fan - favorite. Her mysterious past and her connection to Ganta make her a great character to write about. You could write a story that reveals more about her origin, or a story where she has to make difficult choices that put her relationship with Ganta at risk.