A new character could be the main character as well. This new character could be a human who is suddenly thrust into the vampire world in a 'once upon a time' way. They could be the one to bring a fresh perspective to the story, seeing the vampires and their lore from an outsider's point of view and having to learn quickly to survive.
Perhaps a story where a character discovers a long - forgotten magic that has been dormant since the very beginning of the vampire lore in this universe. This magic could be the key to solving a major problem like a new, more powerful type of enemy that is threatening both the vampires and the humans in Mystic Falls. The character then has to embark on a journey to fully understand and master this magic.
One of the popular vampire characters is probably Killian Jones (Captain Hook). In some fanfictions, he might be turned into a vampire or have some sort of vampire - related story arc. His complex character and his relationship with other characters in 'Once Upon a Time' make him a great candidate for vampire - themed stories.
A character like Hook could be an interesting choice. He has his own complex past and a bit of a rogue nature. Pairing him with Drizella could lead to some interesting power plays and character development. They could either become allies in some devious scheme or Hook could end up being the one to make Drizella see the error of her ways.
A young girl with the power to communicate with animals could be a main character. She might be the one who first discovers the strange happenings in the Golden Green and starts the journey to uncover its secrets.
It could be a young wizard. Wizards are often main characters in fairy tales. They can use magic to solve problems and have exciting adventures.
It could be an evil stepmother figure who is jealous of Snow and Charming's happiness and their new baby. She might try to steal the baby's magic or harm the baby to get revenge on them.
In some cases, it could be a corrupted lost boy. Maybe one of the lost boys has turned bad due to jealousy or a desire for power. He sees the hurt girl as an easy target to assert his dominance or to gain something from her, like her friendship with Peter Pan. So he becomes the main source of trouble for the girl in the fanfiction.
The mermaids could be the main antagonists. They are known to be jealous and territorial in the Peter Pan stories. If the lost girl catches Peter Pan's attention in a way they don't like, they could make her life in Neverland miserable. They might use their alluring voices and their knowledge of the waters around Neverland to try and drive her away or lead her into danger.
A 'found family' trope is quite common. Characters who were alone or estranged in the show form a close - knit family in fanfiction. For instance, Henry might bring together different characters who then become his new family. Also, the 'hurt/comfort' trope, where a character is hurt (physically or emotionally) and another character comforts them, is frequently used in Once upon a Time fanfics.
Family relationships are a big theme. Some fanfics focus on the family dynamics between characters like the Charmings and their descendants. There are also fanfictions that revolve around the theme of identity. Characters in 'Once upon a time' often have complex identities, and fan writers like to dig deeper into how these identities are formed and how they can change over time.