I think that the Author emerging from the portal is bad news for Bill Cipher’s ultimate plan or whatever he’s plotting.



How?

Symbolism of course.



And I’m talking this ominous window in particular.

This Bill-shaped window which made various appearances in the show.

Notably in Sock Opera, which is currently the only Season 2 episode where the triangle demon truly appears.

Fast-forward a couple of episodes, we get to the game-changing Not What He Seems, the beautiful episode that completely destroyed the status quo of the show.



Early in this episode, when the special forces are securing the Mystery Shack, they break a couple of windows, including the rather iconic Bill-shaped one.

Now, since this is Gravity Falls, a show that is full of symbolism, I think this window-shattering scene is probably a metaphor for “not even Bill Cipher could predict this”, and with “this”, I obviously mean the return of

the motherfuc*ing Author, possibly the Yin to Bill’s Yang and the evil illuminati dorito’s arch enemy.



Long story short: with the Author’s return, NOT everything is going according to Bill’s plan and this means that there are things not even the all-seeing triangle-demon can predict. Not What He Seems, as I said, was a game-changing episode for the entire show’s universe, including Bill.



It’s also entirely possible that I’m reading into it a bit too much though.