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

https://www.fanfiction.net/s/13242212/1/The-Monster-I-Could-Become

In the year 20-Snyventy-12, Time Baby sends a squad of his best soldiers, equipped with extremely powerful and dangerous relics and equipment blessed and endowed by various deities, including the Axolotl and the big guy upstairs himself, to capture Alcor out of apparent fear that the demon might one day challenge his rule of the world and take him out. Despite his immense level of power at the time, Alcor found his back against the wall, and in a desperate attempt to escape, the dream demon used his newest and most dangerous power, and attempted to create a feedback loop. Unfortunately, Alcor ends up in the canon universe of Gravity Falls in the form of a sixteen-year old teenage human, despite not losing an ounce of his raw demonic power. Once there, Alcor quickly befriends the Pines family and devises a plan to pose as Stanley Pines’ grandson that he didn’t know about, and takes part in the Pines’ hijinks and misadventures throughout the summer.

Notable Features of this AU are:

• Alcor’s cane sword is modeled after the cane swords from Assassin’s Creed: Syndicate, and has an ebony horned sheep head as a nod to Alcor’s flock from his home dimension, and a black and silver blade.
• Alcor refuses to tell his origins to his younger counterpart, Mabel Pines, Stanley Pines, Stanford Pines, or anyone for that matter, and it isn’t until Dipper and Ford use the Mind Reader during the events of The Last Mabelcorn that they learn the full story of the events of the Transcendence and that Alcor has, from their perspective, been living a fate worse than death, especially after he accidentally killed the Wendy Corduroy of his dimension in a fit of demonic rage and the Mabel from his dimension passed away.
• Due to his accidental transformation into an immortal teenage human (although he later discovers he can change his age at will, thus allowing him to grow old with his beloved), Alcor now believes himself free to engage in romantic and sexual relationships, and actually manages to have one with none other than Wendy Corduroy, which ultimately results in marriage and the birth of Alcor’s son and daughter, Anser and Wenda Pines. Needless to say, once Alcor starts courting Wendy, Dipper and Robbie are NOT happy, and even combine forces to sabotage the relationship.
• Thanks to Alcor’s meddling with their minds, after the events of Double Dipper, Preston and Priscilla Northwest loosen Pacifica’s leash, allowing the heiress to go out into town more freely, wear whatever outfit she chooses (unless it’s for a party) eat whatever she chooses (unless it’s for a party), and no longer force her to obey her bell, thus allowing the real, far nicer and compassionate Pacifica Northwest to shine through her stuck up, snobby, mean girl stereotype to the point where most people don’t even recognize her, which in turn causes a romantic relationship to blossom between her and Dipper Pines. However, despite this, the Northwest patriarchs continue to be the stuck-up, arrogant, and pompous jerks who treat the townsfolk as though they’re something unpleasant on their shoes that they’ve always been. According to Alcor, this was intentional so that Archibald Corduroy aka the Lumberjack Ghost could “still have his fun” during Northwest Fest.
• Despite falling in love with her once again and finally achieving his adolescent dream of marrying and starting a family with her, Alcor is still haunted by the accident that led him to kill the Wendy Corduroy of his home dimension, which led to the creation of his fox familiar, Anser, whom his son is named after, and secretly fears that he could lose control again and kill his bride.