One idea could be a demon OC who was once a human but was cursed into becoming a demon. This demon could be trying to find a way to break the curse while interacting with the main Gravity Falls characters. For example, they might befriend Dipper and Mabel and the trio could go on adventures together to find a magical artifact that could reverse the curse.
A demon OC that is an outcast among the other demons in Gravity Falls. This OC could have a more benevolent nature compared to the other demons. They could be hiding from their own kind while trying to live a normal life in Gravity Falls. The OC might start a business, like a small coffee shop, and the main characters could discover the demon's true identity over time as they become regular customers.
One interesting Gravity Falls OC fanfiction could be about an OC who has the power to control time and how they interact with the main characters like Dipper and Mabel. They might use their power to prevent certain disasters in Gravity Falls or to gain knowledge from different time periods.
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 Reverse Falls accidentally cross over into the Gravity Falls universe. They might initially be seen as threats by the Gravity Falls gang, but then they realize they have to work together to solve a mystery that threatens both universes. For example, a powerful and ancient evil that can only be defeated if they combine their unique skills and knowledge.
Another fanfiction idea is that the demons in Black Butler are somehow related to the strange creatures in Gravity Falls. A fanfic could explore how the Phantomhive family's butler, Sebastian, would handle the mysteries and anomalies of Gravity Falls. He might be intrigued by the Journal and try to use it for his own purposes or to protect Ciel in this new and strange environment.
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 plot idea could be that the characters from Gravity Falls enter the Pokemon world. They could go on a journey to become Pokemon trainers, facing challenges like gym battles and Team Rocket - like villains. For example, Dipper and Mabel could each catch their favorite Pokemon and compete in Pokemon contests.
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.
One idea could be a crossover where Danny Phantom visits Gravity Falls. He could use his ghost powers to help the Pines family solve some of the mysteries there, like dealing with a supernatural entity that's beyond the normal Gravity Falls weirdness. Maybe he teams up with Dipper to fight a ghostly creature that has been causing trouble in the forest.
There's also 'OC Adventures in Gravity Falls'. In this fanfic, the original character teams up with Dipper and Mabel to solve a new mystery that has emerged in the town. It does a great job of integrating the OC into the existing group dynamic, and the plot is full of twists and turns that keep you hooked.
One idea could be a story where baby Dipper and Mabel discover a magical creature in the woods near their home in Gravity Falls. The creature is lost and they decide to help it find its way back. Another could be a fanfic about their first day at daycare in Gravity Falls and all the strange things that happen there. Maybe they meet other kids with unique abilities or find a secret part of the daycare.