A cool plot might involve the Pines family taking a trip to Mewni. Dipper is intrigued by the magical creatures and starts researching them, while Mabel befriends some of the more friendly magical beings. Meanwhile, Star has to deal with the chaos that the Pines family brings, and they all have to work together when an old enemy of Star's resurfaces and targets the humans as a way to get to her.
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.
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.
Definitely Star and Dipper. Star is this wild and powerful magical girl, and Dipper is the curious and knowledge - seeking boy. They could share their knowledge about the different kinds of magic and mysteries in their respective worlds. For example, Star could show Dipper some of her spells, and Dipper could introduce Star to his research on the strange happenings in Gravity Falls.
One possible plot could be that Steven and the Crystal Gems end up in Gravity Falls. They could team up with Dipper and Mabel to solve a mystery that involves both the magic of Gravity Falls and the Gem technology. Maybe there's a Gem hidden in Gravity Falls that has been causing strange phenomena, and together they have to find and deal with it.
I think Dipper and Star would be a really interesting pair. Dipper is all about mystery - solving and Star is this wild magic - using princess. They could learn a lot from each other. Dipper could help Star with his logical thinking when it comes to dealing with some of the more complex magical problems she faces, and Star could show Dipper some really cool magic that he's never seen before.
Well, one idea could be that the characters from Don't Starve end up in Gravity Falls. The strange and magical elements of Gravity Falls would be a huge challenge for the Don't Starve characters who are used to their own, albeit also strange, world. For example, Dipper could try to help Wilson figure out how to get back to his world while they both deal with the mysteries and threats in Gravity Falls.
One possible plot could be that the Gems come to Gravity Falls to investigate some strange energy readings that are similar to their own gem powers. They meet Dipper and Mabel, and together they figure out that there is a connection between the two worlds' magic.
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.
Mabel and Marco could be an interesting pair. Mabel is all about fun, friendship, and having a good time. Marco is more of a practical and responsible character. Their interactions could be really comical as Mabel tries to get Marco to loosen up and have more fun, while Marco tries to keep Mabel in line when things get too wild.
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.