First, you need to understand the two universes well. Know the characters, their personalities, and the rules of their worlds. For Gravity Falls, know about the mysteries, the strange creatures, and the relationship between the characters. For MLP, understand the magic, the different pony types, and the importance of friendship. Then, come up with an interesting concept for the crossover. It could be a simple visit between the two worlds or a more complex situation like a shared threat that forces them to work together.
Mystery is also a big theme. Both Naruto and Gravity Falls have elements of mystery in their own right. In the crossovers, these mysteries blend. For instance, the mystery of the Akatsuki's goals in Naruto might get intertwined with the mystery of the Gravity Falls anomalies. And of course, there's the theme of self - discovery. The characters, whether from Naruto or Gravity Falls, learn more about themselves as they interact with the new and different characters from the other universe.
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.
Another common theme is friendship. The main characters in both shows are young and form strong bonds with their friends. In crossover fanfiction, you'll often see the characters from Hilda befriending the characters from Gravity Falls. They might rely on each other's unique skills and knowledge to get through various challenges, like when Hilda uses her wilderness skills and Dipper uses his mystery - solving abilities to face a common threat.
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.
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.
To create a compelling character interaction, think about the goals of the characters. In a crossover, a character from Gravity Falls might be trying to save their world, while a Reverse Falls character could be trying to take over the other world. When they meet, there will be conflict and negotiation. Let their different motives drive the conversation and actions. Also, use their unique abilities or powers in the interaction. If one can read minds and the other can manipulate shadows, how do those abilities play off each other when they interact? It can lead to really interesting scenes.
One interesting concept could be the characters from Gravity Falls getting Saiyan powers. For example, Dipper could discover he has latent Saiyan abilities and start training like Goku. This would change his dynamic in the Gravity Falls world as he tries to balance his newfound power with the mysteries of his hometown.
Well, to start a Gravity Falls crossover fanfiction, you need to have a good understanding of both the Gravity Falls world and the world of the other fandom. For instance, if it's a crossover with the MCU. Think about how the Avengers would react to the mysteries of Gravity Falls. You can start with a character from one universe stumbling into the other. Say, Captain America ends up in Gravity Falls. Then, develop the story from there, showing how he adapts and how the local characters respond to him.
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.