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.
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.
One interesting concept could be the characters from Gravity Falls getting Saiyan powers. For example, Dipper could discover he has latent Saiyan abilities and start training like Goku. This would change his dynamic in the Gravity Falls world as he tries to balance his newfound power with the mysteries of his hometown.
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.
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 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.
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.
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.
The merging of the two different settings. Gravity Falls has its own spooky and mysterious forest and town, while Undertale has the Underground with all its unique areas like Snowdin and Hotland. When these two settings are combined in a fanfic, it creates a whole new world full of possibilities. For instance, a fanfic might have a scene where the characters are exploring a part of the Underground that has been influenced by the mystery of Gravity Falls. This could lead to some really unique and unexpected story developments. It also allows for a lot of creativity in terms of how the rules of both universes interact.
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.