Mabel and Marco would be a really fun interaction. Mabel's energetic and positive personality would mesh well with Marco's more down - to - earth but still adventurous nature. They could go on crazy adventures together in either Mewni or Gravity Falls, like exploring an old, haunted castle in Mewni or looking for a hidden treasure in Gravity Falls.
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.
Another interesting interaction would be between Bill Cipher and Glossaryck. Bill is this super - devious and powerful being from Gravity Falls, and Glossaryck is this all - knowing and mysterious magical entity from SVTFOE. They could have some really intense mind - games with each other, as Bill tries to outwit Glossaryck to gain access to the secrets of Mewni's magic, and Glossaryck tries to keep Bill in check to protect the magical world.
Definitely Steven and Dipper. Steven is so open - hearted and has a positive outlook, while Dipper is more skeptical and curious in a different way. Their different personalities could lead to some really interesting exchanges as they try to understand each other's worlds.
The interactions between Ford Pines and Danny's parents could be fascinating. Ford, with his vast knowledge of the strange and supernatural, might be intrigued by Danny's parents' attempts at ghost hunting, even if their methods are a bit more primitive compared to his own scientific investigations. Meanwhile, Danny's parents might be in awe of Ford's knowledge and the things he has discovered in Gravity Falls. This could lead to some comical moments as well as some interesting exchanges of ideas about the nature of the supernatural.
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.
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.
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.
Definitely Dipper and Pearl. Dipper is always eager to learn about the unknown and Pearl is full of knowledge about Gem history and technology. Their conversations could be really interesting as they share their insights on different 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.
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.
Grunkle Stan and Toriel could be an interesting pairing. Grunkle Stan is a bit of a con artist but has a good heart deep down. Toriel is a motherly figure. They could have some great interactions where Toriel tries to get Stan to be more honest and kind, while Stan might try to find some business opportunities in the Undertale world. It would be a clash of personalities but also a chance for growth for both characters.
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.