Perhaps a fanfiction could be based on the concept of time travel. A character from 'The Originals' travels back in time and ends up in a crucial moment in the history of 'Once Upon a Time'. They could change the course of events in that world, which then has a ripple effect on their own world when they return. For example, Elijah could go back and prevent a key event that led to the creation of a powerful dark magic in 'Once Upon a Time', and when he comes back, he finds that his world has also changed in unexpected ways.
One idea could be exploring the backstories of the lost boys in more detail. Maybe one of them has a hidden connection to a character from the outside world. Another plot could involve a new villain coming to Neverland who has a different kind of magic that challenges Peter Pan. Also, a story about Wendy's granddaughter returning to Neverland to find a lost family heirloom could be interesting.
Well, you could create a 'once upon a time parody fanfiction' where the magic in the fairytale world is actually just advanced technology that the characters don't understand. For instance, the fairy godmother's wand could be a high - tech device from a more advanced civilization. Also, you could make the villains into bumbling idiots instead of the menacing figures they usually are. This would add a lot of humor to the story.
One idea could be exploring Drizella's backstory before she became so bitter. Maybe she had a true love in her youth that was torn away from her, which led to her descent into jealousy and meanness.
A great idea for an Once Upon a Time fanfic is to give a happily - ever - after to a character who didn't get one in the show. Take Neal Cassidy, for instance. You could write a story where he somehow comes back to life and reunites with his son Henry in a more complete way. Or you could do a time - travel fanfic where a character goes back in time to change a crucial event. This could lead to a whole new chain of events and relationships in the Once Upon a Time universe.
Another interesting plot could be Loki falling in love with a character from Once Upon a Time. Maybe he has a relationship with a princess or a powerful sorceress, and their love story is filled with magic, drama, and challenges as they come from different worlds.
One idea could be a new agent arriving at Warehouse 13 with a mysterious past that turns out to be linked to an artifact that was thought to be lost forever. This new agent might unknowingly hold the key to controlling the artifact's power, causing a stir among the existing agents as they try to figure out how to handle the situation.
A love - interest plot for Gideon would also be interesting. He could fall in love with a character from a different realm within the Once Upon a Time universe. Their relationship would be complicated by the different rules and cultures of their respective realms. They would have to overcome many obstacles, like disapproving family members or the threat of war between their two realms.
One idea could be exploring Gothel's backstory before she became the villain we know. Maybe she had a tragic love in her youth that turned her bitter. Another plot could involve Gothel teaming up with an unexpected character from the Once Upon a Time universe, like a hero who thinks they can reform her. A third option is to create a story where Gothel has a daughter of her own and we see how she raises her in contrast to how she treated Rapunzel.
One idea could be to create a story where a new character, an aspiring actress, befriends Rick Dalton and Cliff Booth. She gets involved in their crazy Hollywood adventures and helps Rick revive his career in an unexpected way, like by introducing him to a new type of movie role that suits his acting style better.
One idea could be a story where in the Once Upon a Time universe, a character gets a magical haircut. Maybe the Evil Queen's hair is cut by a mysterious stranger, and with the loss of her long, flowing locks, she loses some of her power. Her vanity was tied to her hair, and now she has to adapt to a new look and a new level of strength.