Well, it might be Rosalie writing to a human stranger. Rosalie has her own complex relationship with humanity, so she could be writing to a human to express her thoughts or to try and understand humans better. Or it could be a vampire from another part of the world writing to Alice, curious about her visions and her role in the Cullen family within the Twilight fanfiction world.
Well, it could be about a person writing letters to someone they don't know within the context of the Twilight universe. Maybe it's a human writing to a vampire or a werewolf, expressing their thoughts, feelings, or perhaps even their love for the Twilight world and its characters.
Carlisle could be involved in a betrayal story. Maybe he is betrayed by one of his own family members who disagrees with his peaceful co - existence ideology. This could lead to a rift in the Cullen family and force Carlisle to make difficult decisions about how to deal with the betrayal.
It could be the main characters from The Flash series, like Barry Allen. He would probably be very interested in seeing how others perceive his adventures in the fanfiction.
Bella and Edward are often main characters in such fanfiction. Since their relationship is central in the original Twilight series, in fanfiction about sacrifices, they could be the ones making sacrifices for each other. For example, Edward might sacrifice his vampiric nature to give Bella a more 'normal' life if she so desires.
The Volturi leaders also frequently show up in these fanfictions. Aro, Caius, and Marcus. Their actions and decisions can be completely different in an alternate ending. They could be more benevolent or perhaps more devious. Their relationship with the Cullen family and Bella can take on new twists and turns.
Bella herself can also be part of these fanfictions in terms of how she reacts to the muscle growth of the male characters. Her perspective, whether it's admiration, fear, or something else, can add depth to the story. Also, sometimes in unique fanfictions, she might even have some form of power - related muscle growth herself.
The Cullen family members are often main characters. Edward, with his brooding nature and ability to read minds, is likely to be involved. Bella might also be there as she is closely tied to the family. And Carlisle, being the patriarch, often plays a crucial role.
Obviously, Edward himself is a central character as he is the one being wrongly blamed. Bella would also be involved as she is closely associated with Edward in the Twilight series. His vampire family, such as Alice, Jasper, Emmett, Rosalie, Carlisle and Esme, would likely be part of the story too. They would either be trying to help him clear his name or be affected by the false blame in some way.
Bella could also be a main character. She could be a target of the poison or be involved in the investigation. Her connection to the supernatural world in Twilight makes her a likely candidate.
Edward is often involved. Since he's so close to Twilight, he would be there to support her or sometimes be a source of conflict as he tries to help in his own way but might not always understand what she's going through. Bella's family, like Charlie, could also be main characters. Charlie might notice the changes in Twilight and try to figure out what's wrong. And then, her vampire friends like Alice could play a significant role. Alice might use her ability to foresee things related to Twilight's PTSD, either to help or sometimes make things more complicated.