Warning, may contain spoilers
Wanna know what i think? Since Oldman McGucket has been cleared from the candidates of 'Who's the Author of the journals?'
i think it's Vwdqv wzlq eurwkhu, wonder why i think that? Because in the beggining of episode 1 season 2 we see Stan put on a rubber glove with six fingers, meaning that could've been the authors lab and that Stan might have a connection to the author. And that Stan has tried to get the portal fixed for 30 years because the author is stuck on the other side in a dangerous place. that is why the shapeshifter said to Dipper that he'd look like this one he finds the author.
Also i have a theory that the creator of the show is actually going to make Dipper do that exact scream once they cross the portal. yes, i think that Dipper and Mabel are going to find the room and cross the portal.
Although this is just a theory i have and there is one flaw. if the shapeshifter has been trapped down in the bunker for 30 years then how would he know that the author isnt himself? but like i said this is just a theory :3