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.
Maybe the villains from Gravity Falls, like Bill Cipher, somehow manage to cross over into the Rick and Morty multiverse. Rick has to team up with the Pines family to stop Bill from causing chaos across all dimensions. The contrasting scientific and magical elements would make for an exciting story as they figure out how to combine their different ways of dealing with threats.
In a Rick and Morty Gravity Falls crossover fanfic, it could be that Morty discovers a mysterious object during one of their adventures that turns out to be a key to Gravity Falls. They go there and find that the Gravity Falls anomalies are affecting Rick's portal gun. Dipper is intrigued by Rick's advanced technology and they all have to work together to find a way to get Rick's portal gun working again and also solve some of the mysteries in Gravity Falls.
Rick and Grunkle Stan would be really interesting. They're both kind of cynical in their own ways. Rick could be dismissive of Stan's 'hustles' at first, but then they might find common ground in their experiences dealing with the strange and unknown. They could end up swapping stories about the weird things they've seen in their respective universes.
In a crossover fanfiction, Rick could be intrigued by the strange anomalies in Gravity Falls. He decides to study them, dragging Morty along. They run into the Pines family, and Rick starts making deals with Grunkle Stan to get access to some of the more mysterious areas of the town. However, his actions start to awaken some long - dormant and dangerous entities in Gravity Falls that both sets of characters have to deal with.
A possible crossover fanfiction could involve Dipper and Mabel trying to get Rick to help them solve some of the really tough mysteries in Gravity Falls. Rick might initially refuse, but then Morty convinces him. And along the way, they get into all sorts of misadventures with Bill Cipher maybe getting involved and trying to use Rick's technology for his own evil purposes.
Rick and Dipper would be really interesting. Rick's cynical and scientific view would clash with Dipper's curious and somewhat naive nature. Dipper would be both fascinated and a bit intimidated by Rick's intelligence and his access to other dimensions. Rick might see Dipper as a young, impressionable mind that he could either corrupt or mentor in a strange way.
Definitely Rick and Dipper. Rick is this super intelligent, jaded scientist who has seen so much of the multiverse, and Dipper is this young, eager mystery - solver. Their interactions would be really interesting as Dipper would be constantly asking Rick questions and Rick would be either annoyed or reluctantly impressed by Dipper's inquisitiveness.
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.
Well, it could be a plot where the Venture Bros' world of super - science and espionage mixes with Rick and Morty's dimension - hopping chaos. The boys from both shows, Morty and the Venture boys, might be dragged into one of Rick's hare - brained schemes. For example, they could be on a quest to find a rare element that exists only in the Venture Bros' universe, and along the way, they encounter all sorts of wacky characters from both shows.
One possible plot could be that Rick somehow gets his portal gun mixed up with one of Stewie's inventions. This could lead to the Griffins ending up in the Rick and Morty universe. Maybe Peter would be fascinated by all the strange creatures and start trying to make friends with them in his typical clueless way. Morty could try to show Chris and Meg around the multiverse, while Rick and Stewie start arguing about whose technology is superior.
One possible plot could be that Dipper and Mabel visit Beach City while on a road trip. They meet Steven and the Gems. The Mystery Shack's oddities could interact with Gem magic in strange ways. For example, a Gem artifact might be mistaken for a regular Mystery Shack item and cause chaos when activated.