TAU canon hinges mostly on these things:
1. Dipper and Mabel are human-born twins.
2. Dipper and Dipper alone becomes an immortal demon via an absorption of Bill’s powers.
3. Mabel remains human.
4. The Transcendence happens in the process.
(Plus a few other fiddly bits and pieces which are less officially concrete but still exert a pull on our decisions in this area. For instance, we all love Henry too much to easily entertain an AU where…I don’t know, where he doesn’t exist, where he doesn’t meet and marry Mabel, where he’s somehow a horrible person…you know?)
So, when a new AU is born in which, for instance, both twins become demons, we may post mention of it, but we don’t continue to post headcanons for that AU. Instead, it generally splits off to its own blog and tag and circle of discussion.
If the AU in question includes the above points, we’re likelier to entertain it here on the blog, though it’ll still come down a bit to what we think is reasonable overall. Drift is one such AU – they still start out as human-born twins, Dipper still becomes a demon, Mabel still remains human; it’s just some of the details that change, and in an interesting enough way to open some new possibilities for stories.
One exception to this generality might be the dimension-hopping one, since it’s hard to envision a dimension in which dimensional walls are easily breached without it going both ways, you know? I think we mods would like to remain pretty firm on that point, if only because the strength of said walls almost has to be a shared characteristic between these alternate universes.