This started out as a PhantomFalls crossover, then I kept writing and it grew into a SuperPhantomFalls crossover. My bad.
Crossover
The sleepy town of Gravity Falls was just a façade. In reality it was a supernatural hotspot, and of course that meant the Fentons would eventually get curious and come investigate.
There were a few minor hauntings in the town, minor things that Danny's parents were actually able to take care of. But really, other than that, there wasn't much on the ghost end of things.
No, when Danny Phantom took to the sky on their first night in town, he saw things even he, as a ghost, thought were impossible. Minotaurs (or Manotaurs, as they called themselves), strange lake monsters, pixies and fairies, gnomes, witches, and he's pretty sure he also saw a vampire or zombie or something along those lines. The whole nine yards.
It was unsettling.
And then of course there were the twins. Dipper and Mabel Pines were armed with a journal and a grappling hook, and when he first met them at the tourist trap known as the Mystery Shack he'd thought they were just cute kids who had a vague interest in the paranormal. Nothing to worry about.
Boy, was he wrong.
Danny kept catching them wandering off into the woods alone, though sometimes that Soos fellow and even the red-headed cashier Wendy would be tagging along with them. Danny stayed invisible, ready to jump in at a moment's notice, but after just a few days he realized that these kids knew what they were getting themselves into every time they stepped out the door.
They were curious kids, of course, and Danny couldn't blame them. But the fact that they knew how to handle some of these creatures, even if it required looking through that journal briefly – it was kind of surreal, even for his standards.
It was just…weird.
Dipper helped take care of a ghost haunting a mansion, which impressed Danny greatly. He and his parents had been invited to that party, surprisingly, because of their "famous inventions and ties to Vlad Masters," and even though Danny had sensed a ghost he hadn't seen it at all – at least, until Dipper showed up, and actually brought the lumberjack out of hiding. If Danny watched in awe as the twelve-year-old managed to catch the ghost – well, he was technically on vacation. And when the ghost escaped, everyone was turned to wood, and Pacifica came in and opened the gates to the town in order to put the ghost to rest – well, he would have stepped in, but, uh, well, there was no where convenient to change.
The town was certainly strange, something Danny never would have thought possible, even with his years of ghost-catching experience behind him. And it's part of the reason he let the kids do their own thing; they knew what they were doing, while Danny's knowledge was limited to ghosts and ghosts alone.
Or, at least, Danny tried to stay out of it, until gravity failed, and he could feel the dimensions rubbing against each other and tearing into the fabric of reality. When everyone crashed back to earth (and thank goodness his sister and parents were okay, but the rest of the town might not be that well off), Danny's gaze instantly turned toward the direction of the Mystery Shack.
After that, the Fentons decided to prolong their stay to investigate, and Danny discovered demons and alternate dimensions besides the Ghost Zone were also very, very real in the not-so-sleepy town of Gravity Falls.
It wasn't supposed to be a big deal.
They were heading to Gravity Falls for a quick little investigation. The little town had somehow slipped the brothers' notice until now, but now it was a top priority.
And lo and behold, while in Gravity Falls the Winchester brothers met up with the Fenton family, the infamous ghost hunters of Amity Park. They'd had encounters before, and while the Fentons didn't know the first thing about wendigos or werewolves or demons, they had some pretty gnarly weapons specially designed for dealing with ghosts. Apparently, in the past few years they'd even built a portal that led to the Ghost Zone, a realm separate from Hell, Heaven, and Purgatory that was full of ambient ectoplasm, which gave ghosts who resided there long enough unique powers.
But that wasn't what Sam and Dean came for.
No, Gravity Falls was filled with not only ghosts, but every other type of supernatural creature to ever exist – or, well, so it seemed.
And, of course, on their first day there, gravity decided to take a vacation.
They were chatting with the Fentons, having just arrived in town and getting a brief update on what was going on. The family had been contemplating leaving town after having spent just over a week in the town and realizing that all matter of creatures were present in the town, when suddenly people were screaming and their feet were no longer touching the air. Luckily they were indoors; looking out the window, they could see people rising steadily, higher and higher into the air, and yikes if that wasn't going to hurt – or do worse – when (or if) gravity was restored.
Briefly Dean entertained the thought that maybe this was something normal, that the town was called "Gravity Falls" for this very reason, but the terrified townsfolk shot down the idea right after it had formed.
The Fenton's younger kid, Danny, didn't look terrified so much as intrigued and confused, and the same went for the older girl Jazz. Maybe it was because they lived in a ghost-infested town and they were just used to it, because another glance at Jack and Maddie showed only more excitement and intrigue.
They hovered there for a few minutes, bracing themselves against walls and the ceiling until gravity flipped back on and everyone crashed back to earth. And Dean couldn't help but wince at the sounds of heavy cars and trucks crashing back to earth, of roofs falling back onto destroyed houses, of limp bodies becoming pancakes on the pavement. Unsteadily they all rose, and the two brothers watched in curiosity as Danny then bolted out the door, and toward that tourist trap they had seen called the Mystery Shack.
The Fentons muttered something about staying a while longer, but Dean and Sam were breaking into a sprint after the boy, and after a surprised shout from the parents they realized the rest of the family was on their heels.
Fine. Hopefully Danny had seen something and knew what was going on, because they'd barely been there a day and already Dean wanted out of this crazy-ass Oregon town.