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

http://archiveofourown.org/works/4421897/chapters/10046450

Alcor notices that his powers aren't working the way they should so he tries to find out why by travelling through a rip in time and space which leads him to Gravity Falls, just after Fiddleford had been through the portal. He eventually ends up taking Bill's place.

"This was the time paradox. For whatever reason, Bill must have found a plaything more interesting than a stodgy scientist in a backwards town in Oregon, or been caught up in activities that held a more straightforward reward- Alcor could guess what would appeal to Bill in such a manner, had been enticed by many of the same things over the years.

And Bill's absence was breaking down the time stream.

Somebody needed to make Stanford realize how dangerous the portal was, so that events could unfold as they always had 

-the fight, the Mystery Shack, the Transcendence.

Somebody needed to step in .

And there was only one candidate able, if not entirely willing, to step up to the plate."

Transcendence +Relativity falls AU

Relativity falls itself is an AU were the ages of the two paires of twins are switched, thus Stan and Ford are the ones adventuring , and in this case it is Ford who becomes a demon .

• At first, due to their connection as twins, only Stanley can see and hear Stanford. In their teen years, Ford would follow Stan around as a companion and protector. A companion to talk to because he doesn’t like hanging out with other dream demons (or anyone else for that matter) and a protector from the supernatural that would harm Stan and anyone else he encounters. 
• Because it often looks like he’s talking to himself, while talking to his brother, Stan would often be ridiculed and shunned as a ‘weirdo’. Ford sympathizes with Stan, seeing as he was often shunned too for looking different with his six fingered hands, before his time as a dream demon. 
• As they got older, Ford would watch Stan get into numerous relationships, most of which ended badly, and eventually settling down and become a father to 3 kids. Whether he had more children prior to that is unknown even to Ford. ;)
• Stan once tried to ask Ford a favor to use his powers to do weird magic tricks or conduct fake fortune tellings to try to trick people into giving him their money. This did not end well.
• To differentiate from the aliases in the main AU, Stanford and Stanley are not named Alcor and Mizar. Instead Stanford is Táltos (named after a six fingered figure in Hungarian mythology) the Wise/Wanderer  and Stanley is Erélyes  (meaning strong in Hungarian) the Trickster/Thief.
• When not busy protecting/looking after Stan, his descendants, or current incarnations of his friends, you would find Ford reading/researching a book or a scroll all about Dream Demons or the Magical Creatures whether he’s in the Archives of the Supernatural (one of the many Libraries in the Dreamscape) or in a regular library in his physical form. 
• Ford is the kind of person who would work for the government if they needed him, even if it’s morally ambiguous. (He did create those tie prototypes in canon for the “President’s puppet master's" )

False Memories AU

Main article: False Memories AU

Atlantica AU

In which Alcor still exists, but Pacifica also becomes a demon at some point. Her alias is 'Atlantica', which goes with her ocean theme. After becoming a demon, her pro-nat parents practically disown her. Though their initial 'prank wars' are more extreme, Dipper and Pacifica are on good terms, and offer support to each other throughout the centuries. They maintain a flirty, but non romantic sort of relationship.

Strangely enough, she has an odd sort of interest in Mizar (enough so that Dipper feels the need to be slightly protective when she's in his and Mabel's presence).

Within the Twin Souls cannon, she is portrayed as Alcor's crazy ex.

Someone Like Me

Main article: Someone Like Me AU

Broken Prophecy AU

In which the Transcendence occurs during Wierdmageddon; it is, in effect, what stops it.

The prophecy came true, but not in the way anyone expected. In canon, perhaps it was Stan's refusal to complete the wheel that saved all their lives. In this AU, he did not refuse to take Ford's hand and that is how they destroyed Bill and shoved Wierdmageddon back through the rift. However, in the process of doing so, they also destroyed the boundary keeping the magic within Gravity Falls.

Bill was in the physical plane at the time, so at his demise the energy keeping him alive was released. It created a massive flood of demonic energy in a very small area. Five out of the ten people of the prophecy picked up on this energy, inadvertently absorbed it, and to everyone else, vanished, presumed dead.

And in truth, they did die. But their souls didn't move on. Instead, each one was caught in a web of demonic energy, trapping them in the mindscape. Transforming them into demons.

Story to story, those five demons vary, but most often it's the four Pines and Wendy or Pacifica.

The five stick together at first, defending each other from other demons' attacks, very unlike any other demons, who go at it alone. They have their reason for staying together: they all want the same thing. To get back to the physical plane and show their friends they aren't dead.

To the other demons, they end up with aliases. Dipper and Mabel become Alcor and Mizar. Stan and Ford take on the names Alioth and Alkaid, respectively. As the last of the five varies so much, they will remain unnamed here.

Then the summonings start.

Dipper is most often the one pulled away first, but Mabel insisted on coming with him. The summoners are stunned to get two demons instead of one, and it all kinda spirals downhill from there for the cultists who want to destroy the world.

Years later, when the remaining five of the prophecy end up summoning these five new, powerful demons for answers, they discover the five lost friends from so long ago.

And that is the end of the guide. Every story is different, and it would be boring if the AU had to many strict guidelines. Have fun!

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.