PLANES PILGRIM or DREAMLORD or ASTRALWALKER AU
Well ... not an Alternate Universe, per se, since the idea is post-canon. But you get what I mean. Anyway!
The basic idea is that Wirt and Greg, after the events of the series, possess and slowly develop certain powers related both to dreams and to alternate planes of existence. Those powers are: the ability to fall asleep at will, to awaken from a normal sleep at will, to enter planes of existence connected to our own (like The Unknown or The Desert or The Final Catarct or The Giant's Skull or The IKEA Eternal or The Starfield of Anorion, etc.) via their dreams, and to enter and manipulate the dreams of others.
To be clear, this is not to say everything that happened in The Unknown was All Just A Dream(TM); that trope is far too facile (and honestly boring) an interpretation for me, and also is not supported by canon. But it is undeniable, especially given how the brothers can reenter The Unknown in dreams in the comics taking place after the show, that The Unknown and the brothers' dreaming are related. Or, rather, they become related. Specifically because of the brothers passing into and then out of The Unknown via near-death experience.
People who can do this are sometimes called "Planes Pilgrims" (or "Pilgrims" for short). But also "Dreamlords" and "Astralwalkers" (also "Walkers" for short) depending on which plane they travel to. Because of their connection to "reality"--the main expanse of the fabric of space-time, of which the others planes are the fraying fringes of that fabric--they also possess the ability to act upon and cause changes to those planes. Otherwise, they tend to remain more-or-less timeless and essentially unchanging, without advancents or progress moving their world forward nor regressions or devolutions dragging it back (The Unknown, for example, if left to itself will always be vaguely and surreally like New England in the early 1800s) (The Beast is dead after the brothers' first misadventure because of their intervention, but otherwise would've always been a part of it).
So they get to explore more weird worlds and have more weird adventures! Yay! Especially as they gain more control over their powers with time, and thus more volition over when and where they'll go. Also who, if anyone, will accompany them. Sara was drawn unintentionally into The Unknown, I believe, by Wirt's desire to share some time with her. Later on, he'll be able to do such things intentionally. (Though accidents will always be a part of it, and "reality" and "surreality" can have a will of their own. It won't ever have a 100% guarantee.)
Upsides of these powers: No more insomnia ever again, and usually no more need for an alarm clock. And there are experiences in worlds beyond imagination!
Downsides: There are experiences in worlds beyond imagination, and that isn't always a pleasant or good thing. Besides, while dying in a dream has no real-world consequence, dying in one of these planes will kill your soul, thus your body will remain unresponsive in a supernatural coma until it eventually perishes. There is genuine danger to the soul, and to the body that can't reawaken as if from a normal, physical sleep.
Lighter uses of these powers: Fun times and new friends (especially since time moves differently, and weeks or months can be lived in the space of a real-world sleep). Or, when entering someone else's dreams, the ability to learn more about them and thus help their psyche. Even reawaken them from a coma.
Darker uses, since any power can be put to a dark use: Forcibly drag someone into another plane to banish them there (thus putting them into a supernatural coma). Torrment someone with nightmares, or spy on them in the most private and vulnerable of position. Good thing Greg and Wirt would never do such a thing, right? Unless ...
Anyway, just an AU idea that's been ricocheting around my head like an old screensaver.
