Weirdmaggedon III is coming quick and I can predict what will be going on. Below are some predictions of what could be happening in the next episode.

-Blendin Blandin is the person who formed the forcefield
•Blendin is being hunted down by Bill's minions and goes to Dipper and Mabel for assistance

-Grunkle Stan dies (of course, this has been foreshadowed throughout the show)

-The Manotaurs and MultiBear will have an epic fight scene together.

-Fanfictions will be perfectly lined up
•Dipper looks into Pacifica's eyes...
     •Mabel walks in...
     •GASP!!

-Mabel will get a boyfriend (at the end of the episode)

-The Gobblewonker will finish it all!
•(spoiler) Let's have this ending Jurassic World style!

-Bill either wants to find a way to destroy the forcefield OR go back in time to stop the forcefield from being created.

-Rumble McSkirmish will make more video-game references
•BONUS QUESTION: Will there be a first-person type of fight scene or more of a classic-arcade fight scene? You can interpret "classic-arcade" however you like.

-Bill's minion, Keyhole, has a connection with the President's Key.
•As theorized before, he can either be stopped by it.
•OR Keyhole will join the Pine Rebellion in exchange for the Key.

-There'll be a moment for Dipper and Mabel to celebrate their birthday.
•They'll be technically teens!

-The truth of how monsters ended up in Gravity Falls

-Gompers is related to Bill somehow
•Is the Square-inati going to reveal itself?

-Gold will stop Bill
     •Stan and Bill said to go get gold. Why not?

-Everything will return to normal
•Well, in Gravity Falls terms.