Another idea in the fanfiction could be that Gravity Falls has a hidden connection to the wizarding world. Maybe there are witches and wizards who have been secretly living in Gravity Falls for centuries. Dipper and Mabel could discover this connection and start learning magic from them. They could then use their newfound magical abilities along with their existing skills to explore the mysteries of both Gravity Falls and the wizarding world.
The crossovers where the Avengers have to deal with Gravity Falls' supernatural villains are really interesting. For instance, when they face Bill Cipher. It's fascinating to see how the Avengers' powers stack up against such a powerful and otherworldly foe.
One interesting crossover could be the characters from Gravity Falls and Steven Universe teaming up to solve a mystery that involves both the Gem world and the strange happenings in Gravity Falls. For example, Dipper and Steven might combine their knowledge of the supernatural to figure out a new threat.
Another plot could involve Harry Potter himself. After defeating Voldemort, he feels a strange pull towards Gravity Falls. There, he meets the Pines family and discovers that there are new threats lurking in this strange town. Harry uses his skills as a wizard, like his Patronus charm, to fight off the supernatural entities in Gravity Falls, while also learning about the unique magic that exists there.
One interesting crossover could be the characters from Gravity Falls ending up in the world of MLP. Maybe Dipper and Mabel could meet Twilight Sparkle and her friends. They could go on adventures together, like exploring the Everfree Forest in MLP's world. Dipper's love for mysteries might lead him to study the magic in Equestria.
One great Gravity Falls and Harry Potter fanfiction is 'The Mystery at Hogwarts Falls'. It combines the magical elements of Harry Potter's Hogwarts with the strange and mysterious happenings in Gravity Falls. The characters from both universes interact in really interesting ways, like Dipper and Mabel exploring the secrets of the Hogwarts castle.
Some crossovers focus on the character relationships. For instance, Wendy from Fairy Tail and Mabel from Gravity Falls could become fast friends. They both have a sense of adventure and a kind heart. Wendy could show Mabel some of her sky dragon slayer magic, and Mabel could introduce Wendy to her world of sweaters and boy bands.
Fanfiction.net is also a good place to search. There are many writers who love to experiment with crossovers, and you're likely to find some interesting Gravity Falls and Harry Potter fanfictions there. However, the search function might be a bit less refined compared to AO3, so you might have to dig a bit deeper. Check the crossovers section and look for tags related to both shows.
There's a fanfic where the Cipher Wheel from Gravity Falls is found in the Room of Requirement at Hogwarts. This fanfic plays on the idea of magic being different in both worlds but still having some sort of connection. It's really interesting to see how the characters from Gravity Falls react to the more structured magic system in Harry Potter. It also has some great action scenes when they have to protect the Cipher Wheel from dark wizards who want to use its power.
One interesting Gravity Falls fanfiction could be those that explore the further adventures of Dipper and Mabel after the events of the show. For example, a story where they discover a new mystery in Gravity Falls during their next summer vacation. Another could be a fanfic that delves into the backstory of Bill Cipher and his connection to other characters in a more detailed way. There are also fanfictions that focus on the relationship between the characters, like how Dipper and Wendy's friendship might develop further.
Well, in some fanfiction, we might see Anna and Kristoff visiting Hogwarts. They could be amazed by the moving staircases and magical creatures. In reverse, Harry, Ron and Hermione could end up in Arendelle. Hermione might be intrigued by the way Elsa controls her ice powers and try to find a scientific or magical explanation for it. Also, the Dementors from Harry Potter could be compared to the dark forces in Frozen 2, like the mist that Elsa has to face.