Sub-AU

Yes, this fandom is really that crazy.

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.

Equivalence AU

In this AU, Mabel is the one who became the demon, while Dipper remains human.

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 in-tact, 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 less 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 into their lives (which couldn't be further from the truth).

In both iterations of this AU, the years where 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.