One idea could be that Dipper and Mabel end up in Equestria. Dipper might be intrigued by the magic there and start studying it, while Mabel could make friends with the ponies easily. Maybe they help the ponies solve a mystery that's been plaguing Equestria, like strange disappearances. Another idea is that a pony from Equestria somehow travels to Gravity Falls. It could be Twilight Sparkle, who is amazed by the different kind of 'weirdness' in Gravity Falls compared to Equestria.
Well, a great concept for a crossover fanfiction could be that Bill Cipher gets interested in Equestria. He tries to manipulate the ponies and their magic for his own gain. The ponies, with their strong sense of friendship, have to team up with the Pines family to stop him. Or perhaps the Cutie Mark Crusaders end up in Gravity Falls. They'd be in for a wild ride, exploring the mysteries and meeting strange creatures. They could learn a lot from Dipper and Mabel about being brave and resourceful in the face of the unknown.
One idea could be a Gravity Falls and Steven Universe crossover. Dipper and Mabel could meet Steven and the Gems. Maybe they'd team up to solve a mystery that involves both Gravity Falls' strange phenomena and the Gem-related threats. It would be cool to see how the different magical elements interact.
One idea could be that the characters from Rugrats somehow end up in Gravity Falls. Maybe the babies follow a strange glowing bug that turns out to be a magical creature from Gravity Falls. And Tommy could befriend Dipper, and they start exploring the mysteries of Gravity Falls together. The Rugrats' naivete could add a new comical element to the Gravity Falls mysteries.
One idea could be that Dipper and Mabel from Gravity Falls end up in the SVTFOE universe. They could team up with Star to battle some sort of interdimensional evil that threatens both their worlds. Maybe they discover a connection between the strange creatures in Gravity Falls and the magic in SVTFOE.
In a crossover fanfiction, Rick could be intrigued by the strange anomalies in Gravity Falls. He decides to study them, dragging Morty along. They run into the Pines family, and Rick starts making deals with Grunkle Stan to get access to some of the more mysterious areas of the town. However, his actions start to awaken some long - dormant and dangerous entities in Gravity Falls that both sets of characters have to deal with.
Perhaps there is an ancient evil that threatens both Gravity Falls and Hilda's world. The characters need to combine their unique skills and knowledge. Dipper's knowledge of the strange and supernatural from Gravity Falls, along with Hilda's experience with the magical creatures in her world, would be crucial. They could start by gathering allies from both worlds, like Wendy and David. Then, they embark on a journey to find the source of this evil and a way to stop it. This would involve a lot of exploration, mystery - solving, and character development as they learn from each other.
One idea could be that the characters from Gravity Falls end up in the FNAF world. Dipper might be intrigued by the mystery of the haunted animatronics and try to solve it using his journal. Mabel could befriend the more friendly - looking animatronics, like Chica, and try to make them cute accessories. Another plot could involve the FNAF villains getting loose in Gravity Falls and the Pines family having to use the town's unique magic to stop them.
One idea could be that Dipper and Mabel end up in the Undertale universe. They might meet Sans and Papyrus. Dipper could be intrigued by the magic system in Undertale and try to study it, while Mabel would probably befriend the friendly monsters right away. Maybe they get involved in the conflict between humans and monsters in the Undertale world and use their Gravity Falls knowledge to find a new solution.
Another theme is the use of magic and the supernatural. My Little Pony has a lot of magic, and Gravity Falls has its own share of supernatural elements like the Journal and various monsters. Fanfictions may combine these aspects, like a story where the magic from Equestria interacts with the strange phenomena in Gravity Falls. This can lead to all sorts of interesting plot developments, such as the ponies using their magic to help fight off the creatures in Gravity Falls or vice versa.
Many of these fanfictions also explore new themes that neither of the original shows might have delved into deeply. For instance, how the Gravity Falls anomalies might affect the pony world in ways that are not just about solving a mystery, but also about changing the relationships between ponies. It can be about the ponies learning to deal with a more 'human' or 'weird' kind of situation, which is a fresh take on both universes.
Another idea is a crossover of the Upside Down and the Weirdness of Gravity Falls. Imagine Eleven using her powers in Gravity Falls and discovering new and strange things about the town. The kids might explore areas where the laws of physics seem to be a mix of both their worlds. For example, they could find a portal that leads to a place that has elements of both Hawkins and Gravity Falls.