Sub-AU

Yes, this fandom is really that crazy.  AUs of AUs of AUs. We have reached inception level and passed it.  

The AUception cannot be stopped.  

Friends with Tax Benefits (FwTB) AU

Main article: Friends with Tax Benefits AU

Drift AU

Main article: Drift AU

Friends with Tax Benefits AU happenings + Drift AU

Yup, we're crazy. We are mixing AUs of AUs to create AUs of AUs of AUs.

Two possibilities here thanks to the Drift AU split!

Bad end drift au: Dipper loses Mabel only a few years after taking her soul to save her. He very likely spirals further into his demonic side, driven by the pain of having lost his sister after so much effort spent to reconnect with her. He’s already pretty much chaotic evil, and Henry doesn’t trust him at all, not even around the kids. Without Mabel there to hold them together, they most likely wouldn’t stick with each other by mutual choice.

For that matter, it’s possible that Willow doesn’t survive the birth either. Without the trust of Henry or Stan, there might not be the deal to save her. Or maybe there is, but it’ll cost a lot more than a year of life between them, and when Mabel dies what little trust they placed in him for that deal is killed as well.

Either way, there wouldn’t be any kind of marriage for tax benefits. Henry might even try to keep Dipper away from himself and the kids entirely - wards, exorcisms, barriers, demon traps, whatever he can possibly think of. Dipper still gets through and mocks the attempts. He may have lost Mabel, but the kids remain - remnants of her, and so naturally they belong to him.

Good end drift au: Dipper is devastated by losing Mabel - it might not be worse than in regular fwtb, but there’s a bit more piled on there, what with being separated from her for all those years, walking the knife’s edge between growing in power and keeping himself, and then only getting a few brief years with her before she’s lost again - for good this time.

But in this version, Henry would trust him. Dipper’s softer, more careful about his demonic nature. He makes more of an effort to be a good person in every way, because that’s all he has to stave off what he’s become. Friend-marriage for tax benefits is far likelier in this version, and they’d be able to mutually support each other in raising the kids and dealing with their grief over losing Mabel.

Equivalence AU

Main article: Equivalence AU

Demonic twins AU

An AU in which both Dipper and Mabel become demons. This AU takes from both the original Transcendence AU , as from the Equivalence AU.

Since the energy is split between the two of them, they’re both a lot weaker at first. They can’t affect the mortal plane outside of summons, and they can’t break out of a summoning circle without some sort of an energy source and since there is no twin in the physical plane who can automatically see them , it takes them years to be able to contact their family . In this aspect it resembles the Drift AU but with the difference that they have each other and thus at least don't suffer from loneliness. From here this story can also takes a split, even if not as drastic as the Drift AU split . In that time of only having each other they could try to keep each other human or they could try to help each other accept that they have become demons and thus need accept their demonic tendencies. They want to help each other but we can't say exactly HOW they help each other . Maybe they mix both

The TWIN STARLETS AU is the good split. It can be found here

Avarice AU

The AU in which Grunkle Stan (Stanley) became a demon instead of Dipper.

Wayward souls AU

In which Gravity Falls is sentient (and sapient) and saves Dipper's life when Bill tries to posses him during the start of the Transcendence. Rather than becoming a demon, Dipper becomes a living bridge between the mindscape and the waking world, and their respective  somewhat-benevolent forests. The results are, initially, a highly spatially-aware mostly-human twelve-year-old with a very green thumb and an impressive repertoire of magic tricks. Then things steadily get worse. 

(AU is which dipper turns into something more like Willow and Henry instead of a demon)

http://archiveofourown.org/series/333829

Loophole AU

Main article: Loophole AU

Transcendence +Relativity falls AU

Main article: Relativity AU

False Memories AU

Main article: False Memories AU

Atlantica AU

Main article: Atlantica AU

Someone Like Me

Main article: Someone Like Me AU

Broken Prophecy AU

Main article: Broken Prophecy AU

THE OFFER ACCEPTED

Based off of a previous canon-compliant one-shot called 'The Offer'. In the original, a demon offers Dipper a deal that he refuses after a flash of insight as to what accepting would ultimately mean.

In this version, he doesn’t get that insight. He has to live it.

In which Mizar is reborn as Alcor's actual freaking demon daughter and it ends horribly for everyone involved.

FEEDBACK AU

Main article: Feedback AU