One idea could be that the ponies from MLP accidentally end up in Gravity Falls. Twilight could be intrigued by the strange anomalies in Gravity Falls and try to study them with Dipper's help. Maybe Pinkie Pie's party - planning skills could be used to liven up Gravity Falls' rather spooky events.
Well, perhaps the villains from Gravity Falls, like Bill Cipher, try to take over Equestria. The Mane Six would have to use their friendship magic to stop him. Rainbow Dash could use her speed to outwit Bill's minions, while Rarity might use her charm to deceive them. And Fluttershy could communicate with the strange creatures in Gravity Falls to get some inside information.
A really cool plot might be that the forces of Hell in Supernatural are trying to take over Gravity Falls because of its unique supernatural properties. Sam and Dean, along with Dipper and Mabel, would need to use their combined knowledge and skills. Sam's research abilities and Dean's combat skills, along with Dipper's knowledge of the town's secrets and Mabel's out - of - the - box thinking, to stop the demonic invasion.
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 interesting crossover could be the characters from Gravity Falls ending up in the world of MLP. Maybe Dipper and Mabel could meet Twilight Sparkle and her friends. They could go on adventures together, like exploring the Everfree Forest in MLP's world. Dipper's love for mysteries might lead him to study the magic in Equestria.
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.
The colorful and magical world of Equestria from MLP could be juxtaposed with the spooky and mysterious Gravity Falls. You could have parts of Gravity Falls, like the Mystery Shack, appear in Equestria. Maybe the ponies think it's a new kind of magical building. Or, have the Gravity Falls forest start to show up in Equestria, with its strange creatures and anomalies.
One great example could be a story where the characters from Gravity Falls end up in the world of MLP. Maybe Dipper and Mabel meet Twilight Sparkle and they go on an adventure to solve a mystery that involves both the strange elements of Gravity Falls and the magic of Equestria. Another example might be a crossover where Bill Cipher somehow gets into the MLP universe and tries to cause chaos, and the ponies have to use their friendship magic to stop him.
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.
A possible plot is that the 'Gravity Falls' characters accidentally open a portal to the world of 'The Replacements'. The Replacements' characters then have to adjust to Gravity Falls' weirdness. There could be a conflict where the Gravity Falls' villains try to take advantage of the new arrivals, but they all end up working together to save both their worlds.
Perhaps the character interactions are a key concept. Imagine Dipper from Gravity Falls meeting Star from SVTFOE. Dipper's inquisitive nature and Star's bold and impulsive personality could lead to some really fun and unexpected adventures. They might team up to solve a mystery that involves both the Gravity Falls' mystery and the magic of SVTFOE.