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.
In a crossover fanfiction, the interactions would be really interesting. Ford, if he were involved, would be very intrigued by the animatronics. He might study them to see if they are related to any of the other interdimensional beings he has encountered. Dipper would be the voice of reason, warning everyone about the potential danger. Mabel would try to find the good in the animatronics, perhaps believing that they are just misunderstood. Soos would be the comic relief, making jokes about the animatronics' appearance. Wendy would be the tough one, ready to fight if necessary. The animatronics themselves might be confused by the Gravity Falls gang. The ones that are more aggressive might see them as a threat, while the more docile ones might be curious about these new visitors.
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.