Character interactions are key. You want to see how the characters from different shows blend together. For example, if it's a crossover with Scooby - Doo, seeing how Shaggy and Dipper would react to the mysteries in Gravity Falls. Also, a good plot that makes sense for both universes. It can't just be a random mash - up.
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.
Sure. One great Gravity Falls crossover fanfic is 'Gravity Falls and Steven Universe: Crystal Falls'. It combines the mystery of Gravity Falls with the magic of Steven Universe in a really interesting way. The characters interact in a believable manner and the plot is engaging.
One possible plot could be that Dipper and Mabel visit Beach City while on a road trip. They meet Steven and the Gems. The Mystery Shack's oddities could interact with Gem magic in strange ways. For example, a Gem artifact might be mistaken for a regular Mystery Shack item and cause chaos when activated.
One main theme is the clash of different worlds. 'Worm' has a more urban, superpower - filled setting, while 'Gravity Falls' is a small, mystery - ridden town. When they cross over, it's about how these two very different environments and the characters within them interact.
Mabel would be excited to meet the Winchester brothers. She might try to get Dean to tell her all his hunting stories and would probably try to knit them some cool accessories. She could also introduce Sam and Dean to Waddles. Dipper would be a bit more cautious around them at first, but as they started working together, he would respect their skills and knowledge. The characters would need to learn to trust each other quickly since they come from different worlds but are facing similar threats.
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 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.
The colorful and magical world of Equestria from MLP could be juxtaposed with the spooky and mysterious Gravity Falls. You could have parts of Gravity Falls, like the Mystery Shack, appear in Equestria. Maybe the ponies think it's a new kind of magical building. Or, have the Gravity Falls forest start to show up in Equestria, with its strange creatures and anomalies.
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.