Drift AU
Drift AU is a sub-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.
Bad end
In the bad end route, Dipper becomes more demonic and loses much more of his humanity. The grief over not being able to tell his loved ones he's alive is debilitating, and to cope with it, he allows his demonic urges and instincts to surface and take control. At first, he's revolted with what he's becoming, but then he decides that "this is just what demons do".
This behavior makes it much harder for anyone (even Mabel) to trust him once he does gain enough power to be present in the physical plane. It doesn't help that they learn he isn't above seriously hurting them in deals if it meant he gained something, and in fact many residents in Gravity Falls learn this the hard way and bear mental, emotional, or physical scars from their exchanges with him. However, 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 infodumps in the main canon.
The relationships Dipper maintains with the Pines family and his friends are much more tenuous than in canon, and most people feel like they are treading on thin ice around him. 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. Despite his behavior, his friends and family make excuses for him, because they can see how much he's suffered and they can empathize with how he was never truly responsible for his drastic change.
Good end
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).
Common elements
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.
