Okay. Normally I don’t make posts like this, but I have a Gravity Falls theory that I need to get off my chest. Forgive me if someone has already made a similar post.
I think the “substantial sacrifice” the Pines family is going to make in Weirdmageddon III is the Mystery Shack.
I got this idea while thinking about the poster given out at the 2015 Comic Con. (Click the link to take a look.)
The top half of the image is clearly the Mystery Shack, right? But it’s been transformed into something living. It has teeth, a tongue, an eye, and a heart. There’s a gold pyramid inside - with a top hat. I assumed this was just the artist’s interpretation, but I’m wondering if there’s more to it than that.
Bill has been turning inanimate objects into monsters. He turned the town water tower into a monster as well. If the unicorn hair barrier around the Shack gets broken somehow, he could possibly turn the entire Shack into a monster - and the Pines might need to destroy the Shack in order to defeat Bill.
This would constitute a “substantial sacrifice” for the Pines family. What does Stan care about most besides his friends and family? His business. Running the Mystery Shack represents something he was finally good at after years of failed attempts. What does Ford tell Stan to give up? His house, the place where all his research took place. And for Dipper and Mabel, the Shack is home base for all their summer adventures.
By giving up the Shack, both Stan and Ford would be letting go of their pasts in a way, allowing them to start with a clean slate. One thing standing in the way of their reconciliation (i.e. the argument over which brother keeps the Shack) would be gone. They’d be free to either rebuild a new Shack together or rebuild the Stan O’ War and leave Gravity Falls.
Also, the Mystery Shack is a kind of character itself. It’s been in every episode in the series. I’m not saying destroying it would count as the rumored character death, but it would certainly have the same emotional impact without killing off any main characters.
Or, you know, I could just be crazy. *shrugs*