Maybe there's a tear in dimensions and then we get to see reverse counterparts.
One day it ends up like the Monster Falls au: Everyone turns into a mythical humanoid creature that best matches their personality (the ones who weren't human turn into humans, Bill ends up getting his own human body and had to be forced to ditch the Hypnos kid. He was actually kind of glad because now he could get away with a lot more... If only the Hecate kids and Hazel didn't set magical chains on him that prevented him to use his magic. So he's still stuck at the camp.)
I have no idea what else to put on here, because I'm not really that creative when it comes to thinking up a story on my own so I'll just leave the rest to you guys!
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Demigod Twins AU
FanficBasically headcannons about the Mystery Twins (and others) as demigods. Gravity Falls (And characters) (c) Alex Hirsch Percy Jackson/Heroes of Olympus (And Characters) (c) Rick Riordan