One common theme is the exploration of different mythologies. Both shows have rich mythologies, and in fanfiction, authors often combine or contrast them. For example, the fae lore from 'Lost Girl' and the fairytale magic from 'Once Upon a Time' are often blended to create unique storylines.
One popular 'Lost Girl Once Upon a Time' fanfiction is 'The Unlikely Alliance'. It focuses on a crossover between the two shows where the main characters from each have to team up to fight a common enemy. The story does a great job of blending the unique mythologies of both shows.
One plot idea could be a story where the lost girl discovers a hidden realm within the Fae world that holds the key to a long - forgotten power. Another could be a love story between the lost girl and a character who is initially her enemy but then they realize they have a deeper connection. Or perhaps a plot where she has to team up with a group of unlikely allies to defeat a new and powerful threat to the Fae and human worlds.
Bo is the most obvious choice as she is the central figure. Writers often explore her character in depth, her struggle with her identity as a succubus and her place in the Fae world. However, Trick is also a popular character to write about. His wisdom and the secrets he holds about the Fae history can create really engaging storylines. Additionally, Hale, with his musical abilities and his role in the Fae hierarchy, can be an interesting character to focus on in fanfiction. His relationships with other characters, especially his loyalty to Bo and Dyson, can be explored in many ways.
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.
One plot idea could be that a lost girl in Neverland discovers she has a special power that can break the eternal youth spell on Peter Pan. Another could be a lost girl forming an alliance with the mermaids to find a way back home against Peter Pan's wishes. And a third could be a lost girl trying to rewrite the rules of Neverland to make it a more equal place for all the lost children.
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.
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.
Begin by choosing your favorite characters from 'Once upon a time'. Decide if you want to focus on one character's story or a group of them. After that, plan out the plot. You can make a simple outline of the beginning, middle and end. Consider the setting, whether it's the Enchanted Forest or Storybrooke. Also, think about the tone of your fanfic. Do you want it to be light - hearted, dark, or something in between? Once you have these elements in place, just start writing. Don't worry too much about making it perfect at first, you can always edit later.
A prevalent trope is the 'reincarnation' trope. Fans might write about how the characters are reincarnated in different time periods or worlds and how their relationships are rekindled. 'Power - swapping' is another interesting one, where characters exchange their powers and how that affects their relationships and the story. There are also 'secret relationship' tropes, especially for characters who had a lot of chemistry on the show but never got together officially, like Belle and Will Scarlet. Writers create scenarios where they have a secret relationship and all the drama that comes with it.