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.
To start writing svtfoe crossover fanfiction, begin by making a list of the key elements from both svtfoe and the other thing you're crossing it with. Let's say you're crossing it with a magical school setting like Hogwarts. List the magic in svtfoe and the spells in Hogwarts. Then, create characters' reactions. How would Star react to seeing a magic wand for the first time? These details will help you start building your story.
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.
A new character could be created as the main villain. This villain could be a powerful sorcerer who has studied the magics of both Sailor Moon and SVTFOE. He discovers a way to merge the two universes in a way that benefits him but is harmful to everyone else. He might try to control the Sailor Crystals and the magic of SVTFOE at the same time. To stop him, the characters from both universes would have to put aside their differences and fight together.
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 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.
Fanfiction.net also has a good number of SVTFOE crossover fanfics. However, the interface can be a bit more difficult to navigate compared to AO3. But if you spend some time exploring the different categories and using the search filters, you should be able to find some interesting crossovers.
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.
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.