Well, a svtfoe crossover fanfiction could involve crossing it with a fairy - tale based world. For example, Star and Marco find themselves in a world where all the classic fairy - tales are real. They have to navigate through the different stories, like 'Cinderella' or 'Snow White', and use their own skills to either fit in or change the course of the stories. It could be really fun to write how they interact with the fairy - tale characters.
A really cool svtfoe crossover fanfiction concept could be a crossover with a post - apocalyptic world. Star and Marco are transported to a desolate, ruined world. Their usual magical and physical skills are put to the test as they face new threats like mutated creatures and gangs of survivors. They have to figure out how to survive and also if there's a way to restore the world using Star's magic or Marco's resourcefulness.
One idea could be a crossover with another popular animated show like 'Adventure Time'. Star and Marco could end up in the Land of Ooo and have to deal with the wacky creatures there while also trying to find a way back home. Another idea might be a crossover with a superhero universe. Maybe Star's magic could combine with superhero powers in unexpected ways.
One idea could be that the Sailor Scouts somehow end up in the SVTFOE universe. They could team up with Star and Marco to fight a common enemy. Maybe an evil force that combines the darkness from both universes.
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.
One idea could be a crossover where Naruto characters end up in the SVTFOE universe. They could learn about the magic there and maybe help Star with a big threat. For example, Naruto's shadow clone jutsu could be used in a creative way to outwit an enemy in the SVTFOE world.
One idea could be a crossover with another magical-themed show. For example, a crossover with "Winx Club". The characters from SVTFOE could meet the fairies from Winx Club and they could have to work together to defeat a common enemy that threatens both their worlds. This would bring together different magic systems and character personalities.
One idea could be exploring how Dark Marco's powers change his relationships. For example, his friends might be afraid of him at first. Another could be him using his powers to fight new, more powerful enemies in the SVTFOE universe.
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.
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 interesting idea could be a crossover with "Moana". Elsa's ice powers and Moana's connection to the ocean could create a really cool dynamic. Maybe Elsa accidentally freezes part of Moana's ocean and they have to work together to fix it. Another could be with "Tangled". Rapunzel's long hair and her adventurous spirit combined with Elsa and Anna's journey in Frozen 2 would be exciting. Rapunzel could use her hair to help them climb mountains or cross chasms. A third idea is a crossover with "Beauty and the Beast". Belle's love for books and knowledge could help Elsa understand more about her powers and the history of magic in the Frozen world.
A possible plot could involve Star's internal struggle with her feelings for Marco while also dealing with her royal duties. This would add a layer of complexity as she tries to balance her personal life and her role as a princess. For example, she might have to make a choice between attending an important royal event or spending time with Marco when he needs her.
There are fanfictions where an SVTFOE OC is a new student at the school in Echo Creek. This OC could be from a different magical realm entirely and have to adjust to the 'normal' life in Echo Creek while also hiding their true magical nature. They might get involved in the drama between Star and her enemies at school.