Some Reddit communities can also be a great resource. Subreddits related to either 'Once Upon a Time' or 'Harry Potter' might have threads dedicated to fanfiction. You can ask for recommendations there too. Just be sure to follow the subreddit rules when posting or commenting.
Harry Potter himself is always a great character to focus on. His journey from an orphan boy to a powerful wizard is full of potential for new adventures in a fanfiction that combines with 'Once Upon a Time'. Hermione Granger is also excellent. Her intelligence and knowledge of magic can lead to interesting interactions in a merged universe. Another good one is Severus Snape. His complex past and hidden loyalties can be explored further, especially when combined with the fairy - tale elements of 'Once Upon a Time'.
One common theme is the clash of different magic systems. In both universes, magic exists but in very different forms. Fanfictions often explore how the magic from Harry Potter, like spells and potions, would interact with the magic in Once Upon a Time, which is more fairy - tale based. For example, a spell from Harry Potter might have unexpected consequences in the world of Once Upon a Time.
One important aspect is the world - building. Since it's a crossover, the author needs to create a seamless world where the Hogwarts and the Enchanted Forest co - exist. This could involve creating new rules for how magic works when the two worlds collide. A great fanfic also respects the original source materials while adding its own unique twists. It can't just completely change the personalities of the beloved characters from 'Once Upon a Time' and 'Harry Potter' without a good reason.
Combining them could involve using the time - travel or alternate - reality concepts. Suppose there's a magical accident that links the world of 'Once upon a time' to the Harry Potter world. Characters might have to deal with different magical rules. For instance, the magic in 'Once upon a time' might be more fairy - tale based, like true love's kiss having great power, while in Harry Potter it's more about spells and potions. The characters would need to figure out how to use both types of magic to solve problems and save both their worlds from a common threat.
There's also a fanfic called 'Potter in Storybrooke'. In this one, Harry Potter is mysteriously transported to Storybrooke. He soon realizes that his magic works differently there. He teams up with some of the Once Upon a Time characters to solve a mystery that threatens both their worlds.
One interesting plot idea could be a time - travel story where Harry Potter goes back in time to the founding of Hogwarts. He could interact with the four founders and perhaps change some small events that have a big impact on the future. Another idea is a crossover with the 'Once Upon a Time' concept, where the magical world of Harry Potter is merged with the fairy - tale realm. For example, Harry and his friends could meet characters like Snow White or Cinderella and they all have to work together to defeat a new dark threat that spans both worlds.
A plot could be that the Harry Potter characters are transported to the 'Once upon a time' realm. Harry has to help the fairy - tale characters defeat an evil wizard who has stolen the magic from both worlds. They would need to use the magic of Hogwarts and the unique powers of the fairy - tale characters, like Cinderella's glass slipper having some hidden magic.
One popular theme is the exploration of female empowerment. In fem Harry Potter in a Once Upon a Time setting, the story might focus on how a female Harry Potter overcomes the challenges in a magical world with her own unique strength. Another theme could be the reimagining of relationships. For example, how her friendships with Ron and Hermione might change with the gender swap. Also, the theme of self - discovery is quite common. As she navigates this new world that combines the elements of Harry Potter and Once Upon a Time, she has to figure out who she really is and what her place is in this magical universe.
She could be more empathetic as a female. Maybe she uses her intuition more in her magical studies and battles. For example, instead of just charging into a fight like the male Harry, she might first assess the situation with her heightened emotional intelligence.