It's been a hell of a ride. Gravity Falls has finally ended, and the last episode was definitely worth the wait. 'Weirdmageddon III: Take Back The Falls' was emotional and exciting, with every relationship resolved and most plot threads neatly tied up. All those characters you know and love got their moment in the sun, however brief.
In true Alex Hirsch fashion, many of the highlights of the episode were callbacks to hints and Easter Eggs earlier on in the show. And of course, the show would not be complete without a hidden message from Bill Cipher, threatening his return...
He's Not Gone Yet
In an excellent bait-and-switch, Bill's greatest weakness turned out not to be the Zodiac Prophecy, but his gullibility to Stan and Ford's clever trap.
Instead of victory thanks to a supernatural deus ex machina, Grunkle Stan sacrificed his memories to trap Bill Cipher in his own mind. Gravity Falls isn't really about the big mysteries – at the end of the day it's the characters which matter most. And ultimately this ending was far more satisfying than a prophecy coming true (no matter how well the Zodiac was foreshadowed).
Stan and Ford were forced to resolve their issues to save the kids, which was a long time coming. But best of all, Stan beat Bill Cipher at his own game, proving that while Bill might be an all powerful trickster, Grunkle Stan's got the monopoly on sneaky gambits.
This actually leaves the door open for Bill Cipher to return too, especially considering Stan regained his memories. Perhaps the mind wipe wasn't 100% effective, and a small piece of Bill survived?
Ancient Powers & Aztec Gods
While this is possible, there's actually another way Bill could return: a hidden hint which was pointed out by several fans, and helpfully exemplified in this video by EliteMasterEric...
If you play Bill Cipher's final words backwards, he seems to be reciting an incantation to ensure his return to our world.
AXOLOTL - MY TIME HAS COME TO BURN - I INVOKE THE ANCIENT POWER - THAT I MAY RETURN
Many fans have already jumped on the first section of this incantation, connecting Bill to the Aztec god Xolotl. Which makes a lot of sense, considering this was the deity associated with (among other things) twins and monsters.
Put this together with the rest of Bill's backwards incantation, and it definitely seems like he's invoking the power of Xolotl bring him back to our dimension. It's stuff like this that makes me really wish we'd got a Season 3 – imagine the Pines up against an Aztec god!
Having said that, with the first letter attached, "axolotl" could also be a cheeky nod to one of Alex Hirsch's favorite ways to tease his fans: during his Reddit AMA sessions, any question which demanded a spoiler as a reply would be greeted with a picture of the salamander aoxoltol, posted by Hirsch.
It seems safe to say that Bill's return to Gravity Falls is possible, if not likely. And although the show has ended, this might be used for a future plot line: Alex Hirsch told TV Insider that it's "not inconceivable" that we'll one day return to the world of Gravity Falls. This could be in a comic book, TV special episode or, as Hirsch suggests, a robotic live-action reconstruction of the show (which is my personal favorite option).
Yet even without a continuation of the story, this clue that Bill could return is a nice way to conclude the show on an open ended note, leaving the audience safe in the knowledge that the Pines's adventures have only just begun.