A young vampire who was recently turned against their will. They are lonely because they miss their human life and family, and they don't fit in with the other vampires yet. They are constantly haunted by nightmares of their transformation and the life they've lost.
Maybe a werewolf who has been banished from their pack for some reason. They are now alone in the twilight - filled nightmares, trying to survive. They face not only the natural threats of the supernatural world but also the emotional turmoil of being separated from their pack. They might be on a quest to prove their worth and be accepted back into the pack.
It could be a human girl who has the ability to see the twilight world that others can't. This makes her an outcast among humans, and she's lonely. In her nightmares, she witnesses the battles between vampires and werewolves and is terrified. But she also has a strong will to understand and maybe even change this world she's been thrust into.
It could be about a character who feels isolated in the twilight world of nightmares. Maybe they are a new vampire or a human caught in the supernatural chaos, and they struggle to find a connection or a sense of belonging.
It could be Bella. Since she goes through so much heartache in the original Twilight series, it would make sense for her to be the focus of a fanfiction about mending a heart. Maybe after the events with the Volturi or some other new threat, she is left broken and someone new comes in to help her heal.
It could be someone completely new. A character that the author creates to disrupt the status quo in the Twilight universe. This new character might have unique powers or abilities that change the course of the story.
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.
Maybe an adventurous traveler who stumbles upon a haunted place and becomes the main character in 'the nightmare scary story'. This type of character often has a sense of curiosity that leads them into trouble.
Perhaps it could be a minor character from Twilight who somehow ends up in a situation where they are kidnapped by a creeper. This would give the writer a chance to explore that character more deeply and in a completely different context. For example, a character like Jessica from Twilight could be the main character. She's not a central figure in the original story, but in this fanfiction, she could have a whole new adventure.
It could be a new character entirely. Maybe a young vampire who was turned recently and is still adjusting to the 'alive inside' state of being undead. This character could struggle with the morality of being a vampire while also trying to fit into the Twilight world's complex social structure.
An old werewolf who has decided to retire from the pack life and goes on a trip to all the places that were important to the werewolves in the Twilight story. Along the way, they might run into some vampires and have to face old prejudices and form new alliances.
Edward Cullen could be a great main character. Since he is already a part of the Cullen family who loves baseball, he could be the one to hit a crucial home run. Maybe this home run is what saves Bella from some sort of danger during a baseball - related event. For example, the Volturi might be there to cause trouble during a game, and Edward's home run distracts them long enough for Bella to get to safety.
Another option could be Jacob. As a werewolf, he has a connection to the spiritual side of the Twilight world. He might be the one to discover the hauntings first and have to figure out how to deal with them, especially if they pose a threat to his pack or the people he cares about in Forks.