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

An AU in which Mabel made Dipper and Henry promise to get married if she died, which is exactly what she did whilst giving the birth to the triplets. Henry and Dipper marry and thus become "friends with tax benefits" (as their relationship is not in any way sexual or romantic). In this AU, Dipper is even more protective of the children and Henry, and takes care of Henry's reincarnations just like he takes care of Mabel's. Henry become the woodsman much earlier, and at one point even gives Dipper his soul

It also has it's own version of Twin souls

Drift AU

The Drift AU is a darker version of the transcendence AU, in which Mabel, just like everyone else, can't see, hear or touch Dipper until he gains a certain level of power to breach the physical world. In his desperation to do so, he takes on far more deals, and devours more souls - thus making his power grow much faster than in the canon story.

Due to his lack of human interaction, this AU is actually a set of two possible outcomes: a bad end, and a good end.

In the bad end route, Dipper becomes more demonic and loses much more of his humanity. In coping with his debilitating grief, he allows his demonic urges and instincts to surface and take control, rather than letting his human conscious suffer from being unable to tell his loved ones he's still alive. He eventually comes to accept "this is just what demons do" without much of a fight, making everyone, even Mabel, harbor a healthy distrust of him. (It doesn't help that they learn he isn't above seriously hurting them in deals if it meant he gained something - many residents in Gravity Falls learn this the hard way and bear mental, emotional, or physical scars).

The relationships Dipper maintains with the Pines family and his friends is like constantly treading on thin ice. Mabel and Henry don't trust him to be alone with their children (though he's arguably the most gentle with them). He spends less time "playing house" as he puts it, and more time wandering on his own and doing normal demon things. The only reason his friends and family continue to welcome him back with hesitant arms is because they can see how much he's suffered, how he was never truly responsible for his drastic change... and because every now and then, he has episodes where his humanity returns for a short while, and he openly grieves in ways that rival the tantrums that come with "info dumps" in the main canon.

In the good end route, Dipper goes the opposite extreme. It's still heartbreaking, just in a different way.

Instead of coping by losing himself in his demonic urges, Dipper retains his humanity and mourns twice as much. He still gains power faster and agrees to more deals, even goes so far as to devour souls of less savory folk - but only because he's desperate to see his loved ones. But since his humanity is still intact, the guilt he experiences crushes him over the years.

By the time he finally reunites with his friends and family, he's a quieter, more solemn version of his former self. He's hyper aware of how terrifying he is, and is far more self conscious of it. He has far fewer demonic outbursts of rage, and approaches events of conflict with a calm stature - which can inadvertently make him appear even more intimidating. He treats everyone around him like glass, genuinely worried that he could very easily cross some sort of line, and they'd never forgive him and allow him back into their lives (which couldn't be further from the truth).

In both iterations of this AU, during the years in which Dipper is unable to contact anyone, he tries over and over again to reach Mabel, to the point where she thinks she's haunted. At first she wonders if it's Dipper's ghost, but she eventually changes her suspicions to that of a demon, and later, the infamous Alcor.

Just before Dipper finally attains enough power to breach the physical world at will, Mabel gets herself in deep trouble (most likely a cult, as she begins to investigate Alcor and why he could possibly be haunting her) and finds herself mortally wounded. By devouring the souls of those who harmed her, Dipper makes that final push into the physical world just in time to convince a barely lucid Mabel to make the deal that saves her life, and gives Alcor her soul like in the canon story.

Needless to say, this destroys Dipper's plans to reunite with his family. He feels Mabel would never accept him after doing something so horrible to her, but he has little choice in the matter when she eventually summons - and attempts to trap - Alcor for taking her soul. It's during this scuffle that he finally admits to her (as well as his other Gravity Falls friends and family present) his true identity.

Mabel has no doubt in her mind that Alcor is Dipper, due to the bond they share. However, everyone else has a healthy amount of skepticism. But not too far into the future, Stanford Pines resorts to summoning a knowledge demon and is able to work a deal that puts their doubts to rest (at the expense of all his research - he did his best, but he still got screwed over). He resents Dipper for a while, but comes to at least be thankful that everyone can be certain the demon Mabel has welcomed into her home is in fact her long-lost twin brother.

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

In this AU, Mabel is the one who became the demon, while Dipper remains human. There are different canon's as to how this happens, but a commonly accepted one is that Mabel makes her way to Dipper right before Bill enters his body and pushes him out of the way, but is stuck fighting Bill herself. She wins and the plot continues really similarly to the Transcendence AU, but with Mabel. The similarities lie for a couple of years, although switched around, before it starts to really diverge. Dipper marries Jocelyn, who's Lazy Susan's great niece. In this AU, Lazy Susan's closed eye has the sight and is purple. Jocelyn has the same thing in her right eye. The have triplets (Stanley, Tyrone, and Maeve) and, later on, another child (Wendy, after Wendy's disappearance).

A link to more information, and Equivalence AU's own blog is here: 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’s doesn’t like hanging out with another dream demons (or anyone else for the 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 be shun as a ‘weirdo’. Ford sympathizes with Stan seeing as he was often shun 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 them 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 trying to trick people into giving him their money. This did not end well.
• To differentiate from the alias 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" )

.http://transcendence-au.tumblr.com/post/106483715903/what-about-heartwrenching-headcanon-what-if

This ask, as well as the subsequent short fic it spawned, is part of a underdeveloped (and unamed) AU in which Dipper Pines dies when Bill tries to posses him, and Bill merges with his memories. In this alternate universe, Bill actually believes he's Dipper until he encounters the reincarnation of the real Dipper's soul. Currently, the material linked above is all that is available on this sub-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.

https://www.fanfiction.net/s/11376083/1/Someone-Like-Me

This is an AU in which the transcendence never happened, however, Dipper become Alcor because of a broken deal he made with Bill.

The result left Bill at the lowest power level for a demon, and Dipper with almost all of his power. Because of the circumstances of the power transfer, Dipper is prone to more fits of rage, and his demonic form acts almost like a different personality.

To try and stop the power from overwhelming him, Dipper makes another deal with the now severally weakened Bill, in which the triangle would teach him how to control the power, and in exchange, Dipper would take part in deals and such like a normal demon, which he is extremely against in this AU.

This AU only consists of one transcendence fanfiction, which is linked above as this AU is currently unnamed.

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.

Bille 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.