Bill Cipher would be a major wildcard in their interactions. If he were to cross paths with the Winchesters, he would likely try to manipulate them for his own amusement. He might offer them false information or try to pit them against the other characters in Gravity Falls. The Winchesters, being used to dealing with devious creatures, would have to be on their guard and figure out a way to outwit Bill while also trying to solve whatever mystery has brought them to Gravity Falls in the first place.
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.
Mabel and Hilda would be fast friends. They both have a love for the strange and wonderful. Mabel would be excited to show Hilda her sweaters and all the cool stuff in the Mystery Shack. Hilda would share her adventures in Trollberg with Mabel. They might even come up with some new and crazy ideas for adventures together, like having a slumber party in a haunted forest. Wendy would probably be a bit more aloof at first, but as she gets to know Hilda, she'd start to warm up and they could go on some more adventurous outings, like climbing a really tall mountain in Hilda's world.
Well, I think Lance would be really intrigued by Mabel. He'd probably be impressed by her colorful sweaters and her positive attitude. Mabel, on the other hand, would be excited to meet a real - life space hero like Lance. She might even try to set him up with one of her friends in Gravity Falls.
In a crossover fanfiction, the interactions would be really interesting. Ford, if he were involved, would be very intrigued by the animatronics. He might study them to see if they are related to any of the other interdimensional beings he has encountered. Dipper would be the voice of reason, warning everyone about the potential danger. Mabel would try to find the good in the animatronics, perhaps believing that they are just misunderstood. Soos would be the comic relief, making jokes about the animatronics' appearance. Wendy would be the tough one, ready to fight if necessary. The animatronics themselves might be confused by the Gravity Falls gang. The ones that are more aggressive might see them as a threat, while the more docile ones might be curious about these new visitors.
The characters' interactions would be really interesting. For instance, Ford from Gravity Falls, being a scientist, would be fascinated by the magic in Equestria. He might try to find a scientific explanation for it. Fluttershy would be a bit shy around the Gravity Falls humans at first, but would soon open up. The ponies' positive and friendly nature might have an impact on the more cynical characters in Gravity Falls, like Soos. He could learn a lot from their friendship values. Meanwhile, the Gravity Falls gang could introduce the ponies to new things like s'mores and camping.
On the other hand, Sam and Dean's knowledge of the supernatural would be invaluable to Supergirl. They could teach her about different types of demons, how to detect them, and what weaknesses they have. Their skills in using various weapons like the Colt or salt - loaded shotguns could be combined with Supergirl's powers. For instance, Dean could shoot a salt - loaded shotgun at a group of demons while Supergirl uses her super breath to blow the salt towards a wider area, maximizing the effect.
Gibbs from NCIS would probably be very skeptical of the Supernatural elements at first. He's a man of facts and evidence. But as he sees things that can't be explained, he might start to open up. Dean Winchester, on the other hand, would likely be impatient with the slow - moving NCIS investigation process. They would need to find a middle ground to interact well.
Dean would probably be very suspicious of Hannibal from the start. He's seen a lot of bad guys, and Hannibal's calm and collected demeanor would set off his alarm bells. Sam, on the other hand, might be more interested in understanding Hannibal's motives. Hannibal would likely see the Winchesters as fascinating specimens. He might try to analyze their relationship and use it to his advantage.
Sam and Oliver would probably start off a bit cautious with each other. Sam, being the more cerebral of the Winchesters, would be interested in Oliver's methods of protecting the city. Oliver, on the other hand, might be a bit skeptical of Sam's supernatural knowledge at first. Dean would likely bond with Diggle over their combat skills. They'd probably exchange stories about their battles, and Dean might even show Diggle some of his unique ways of taking down enemies. Castiel, with his angelic powers, would be a mystery to the 'Arrow' characters. Felicity might be both intrigued and a bit scared of him, but she'd also be eager to understand how his powers work.
Well, a great plot might involve Bill Cipher from Gravity Falls making a deal with a crossroads demon in Supernatural. This could lead to a chaotic situation where the rules of both universes start to blend. For example, the symbols and spells in Supernatural might start to have an effect on the Gravity Falls' magical creatures, and vice versa.