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.
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.
Definitely Dipper and Mabel from 'Gravity Falls' would be main characters. And from 'The Replacements', Todd and Riley could be the main ones. They would be the ones driving the plot forward as they interact with the new and strange world or characters they encounter.
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.
Perhaps there is an ancient evil that threatens both Gravity Falls and Hilda's world. The characters need to combine their unique skills and knowledge. Dipper's knowledge of the strange and supernatural from Gravity Falls, along with Hilda's experience with the magical creatures in her world, would be crucial. They could start by gathering allies from both worlds, like Wendy and David. Then, they embark on a journey to find the source of this evil and a way to stop it. This would involve a lot of exploration, mystery - solving, and character development as they learn from each other.
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.
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 possible plot could be that Rick and Morty accidentally portal into the Gravity Falls universe. They meet Dipper and Mabel, and Rick's technology starts malfunctioning because of the strange magical forces in Gravity Falls. Morty and Dipper team up to try and fix it while Mabel tries to get Rick to participate in some of her wacky Gravity Falls activities.
One idea could be that Danny Fenton and the Mystery Twins, Dipper and Mabel, cross paths while investigating a supernatural mystery that involves both the Ghost Zone from Danny Phantom and the strange anomalies in Gravity Falls. Maybe a powerful ghost has escaped into Gravity Falls and is causing chaos there, and Danny has to team up with the twins to stop it.
Another plot could involve Harry Potter himself. After defeating Voldemort, he feels a strange pull towards Gravity Falls. There, he meets the Pines family and discovers that there are new threats lurking in this strange town. Harry uses his skills as a wizard, like his Patronus charm, to fight off the supernatural entities in Gravity Falls, while also learning about the unique magic that exists there.