I'm not aware of its popularity. There are so many fanfics out there that it's hard to say without looking at specific fanfic platforms and their statistics.
Since it's a fanfic, it might be based on an existing story where Bella is a character. In this 'bella as the pack's slave fanfic', the author could be taking a different approach to the character. It could show how Bella copes with being a slave. She might endure abuse or find ways to resist in a non - violent or clever manner. It could also be about how the pack views her, whether as just a tool or if some members start to see her humanity (or whatever equivalent in the story's world) and question the whole slave system.
Another theme is adventure. Often, Bella and the pack go on quests or face threats together. They might have to battle a new enemy in the forest or solve a mystery that endangers the pack.
There could also be other important wolf pack members like Sam. He is often the alpha of the pack and his leadership role can greatly influence the story. His decisions about how to interact with Bella and deal with any threats can be crucial to the plot. And then there might be other pack members like Embry or Quil, who can add more depth to the story with their own personalities and relationships with Bella.
Perhaps Bella's authority is demonstrated by her being the one to call meetings and set the agenda. Also, if she has the final say in important pack decisions, like where to relocate or how to deal with a threat, that shows her authority in the fanfiction.
Bella pack fanfiction could be about a group or pack that includes a character named Bella. It might involve adventures, relationships, or special powers within the pack. Maybe it's set in a fantasy world where the pack has to face various challenges together.
Another great one is 'The Pack's Protector, Bella'. This story focuses on Bella becoming a protector within the pack. It has a lot of action and also some really sweet moments of friendship and loyalty within the pack and with Bella.
One idea could be that Bella discovers she has a special power that can communicate with the wolf pack on a deeper level. For example, she can share their memories and experiences, which helps her understand their culture better.
It could also be Embry. He's a bit of an outsider in the pack in some ways, and imprinting on Bella could give him a sense of purpose and belonging that he may have been lacking. His relationship with Bella could start out as a surprise to both of them but then grow into something that has a big impact on the dynamics of the pack and Bella's life.