Maybe it's a side character from the original Twilight books that the writer decides to give more depth to. Like one of the lesser - known vampires in the Volturi coven who has a secret past and motives that are explored in the fanfic.
It's possible that the main character is a time - traveler who accidentally ends up in the Twilight universe. They would have to navigate the strange world of vampires and werewolves while also trying to find a way back home, all while changing the course of the Twilight story in unexpected ways.
There's also a possibility that it could be a self - insert of the author. In this case, the author imagines themselves living in the Twilight world and having the ability to redo certain situations, which would make them the main character driving the new narrative in this fanfic.
Perhaps it's a story that focuses on the afterlife of a character from Twilight. Like, what if a vampire or a human who died in the original story gets a second chance at life in a different form? It could explore how they deal with their new existence and the relationships they form or re - form in this new state.
Arya Stark from 'Fire and Ice' could be a great main character in a 'Fire and Ice Twilight Fanfic'. She has the skills of a trained assassin and a strong will. In the fanfic, she could find herself in the world of 'Twilight' and use her skills to navigate the dangerous world of vampires and werewolves. Maybe she would form an alliance with the Cullens or the werewolf pack and help them in their battles.
I'm not entirely sure specifically as there are many 'Twilight' fanfics. Generally, a 'Twilight My Immortal' fanfic might be about the 'Twilight' universe, perhaps exploring different relationships between the characters like Bella and Edward in new and creative ways. It could also introduce new plot elements or expand on the lore of the vampires and werewolves in the 'Twilight' series.
It could be a new girl in Forks who just moved there. She has this pink fluffy pencil case that she got from her grandmother. And she gets involved in the Twilight world because of the strange things that start to happen with the pencil case.
It could be Ana, the main character from 'Real Women Have Curves'. Imagine her being transported into the 'Twilight' world and having to deal with vampires and werewolves while still holding on to her strong - willed and body - positive self.
It could be a new character, like a young girl who has just moved to Forks and discovers she has psychic powers. She then gets caught up in the existing drama of the twilight world.
Obviously, Bella and Edward would be great main characters for a reunion fanfic. Their relationship was central to the original series, and a reunion could show how their love has evolved over time. They could be dealing with new challenges as a couple, like raising their daughter Renesmee in a world that still has some threats.
Aro could be an interesting inside man. He's a powerful vampire in the Twilight series. If he were to pretend to be on the side of the Cullens while having his own hidden motives, it could create a lot of drama. He could be secretly plotting to take over their territory or gain access to some special power that the Cullens possess.
Sasuke might be the main character. Since he's always on the hunt for power, he might come across this strange mask. And when the mask starts to 'eat' his face, it would be a test of his willpower and his ability to handle the unknown and dangerous forces that come with it.