Maybe in this fanfiction, the two versions of Dipper and Mabel meet. The Gravity Falls Dipper and Mabel are shocked to see their darker counterparts. They could have a series of adventures where they learn from each other. The Reverse Falls characters might show the Gravity Falls ones a different way of looking at the mysteries and secrets of Gravity Falls, leading to new discoveries.
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.
One common theme is the clash of good and evil. Since Reverse Falls is often depicted as a more sinister version of Gravity Falls, when the two meet, there's a lot of conflict between the good-natured characters from Gravity Falls and the more devious ones from Reverse Falls.
Definitely Dipper and Mabel from both universes. Their different personalities in Reverse Falls and Gravity Falls make for really interesting interactions. Bill Cipher is also a popular choice. His chaotic nature can add a lot of drama, especially when he interacts with the characters from the other universe.
One great 'Gravity Falls meets Reverse Falls' fanfiction is 'The Alternate Encounter'. It has an interesting plot where the characters from both universes interact in unexpected ways. The author does a great job of capturing the essence of both Gravity Falls and Reverse Falls.
One interesting storyline could be the characters from both universes teaming up to solve a mystery that spans across their respective worlds. For example, Dipper from Gravity Falls and a character from Relativity Falls might work together to figure out a strange phenomenon that is affecting both places.
One of the most interesting plot twists could be when the characters from Gravity Falls and Reverse Falls have to team up against a common enemy they didn't expect. For example, a new supernatural force that threatens both their worlds. This would force them to put aside their differences and work together, which is a big change considering the nature of the two settings.
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.
It's a unique take on the Gravity Falls universe. It likely involves an alternate version of the story, with different character dynamics and plotlines in a visual novel format.
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 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.