oK so in The Curse of the Time Pirate's Treasure, Alex H. mentioned that although the whole story was very non canon, there was ONE important canon part. So in the story Dipper and Mabel meet a giant talking Axolotl and they ask it what it knows about Bill Cipher. And it replies in poem -
Sixty degrees that come in threes,
Watches from within birch trees.
Saw his own dimension burn,
Misses home and can't return.
Say's he's happy, he's a liar.
Blames the arson for the fire.
If he wants to shirk the blame,
He'll have to invoke my name.
One way to absolve his crime.
A different form, a different time.
This points to the fact that Bill's dimension got burned down. Or, rather, got burned by himself. ( absolve his crime + saw his own dimension burn ) It also suggests that Bill could be back in a different form and a different time.
Evidence no. 2;;
Bill Cipher gets dissolved by a memory gun in Weirdmaggedon: Part 3. Upon burning up, he turns into several different pyramid-based forms and starts screaming gibberish -- but when the gibberish is reversed, it appears to be "A-X-O-L-O-T-L! MY TIME HAS COME TO BURN! I INVOKE THE ANCIENT POWER THAT I MAY RETURN!" Going back to the line about "invoke my name", this means that Bill could very well return... possessing someone else.
Evidence no. 3;;
After Stan's mind gets wiped, Mabel gets his memories up and running again using her scrapbook full of summer photos. Which means once his mind is wiped, not EVERYTHING is gone completely... if you haven't guessed it by now, that means Bill could still have been recovered in Stan's mind. Which means he could very well be possessing Stan by now...
Evidence no. 4;;
For those who participated in the #FLSKHUKXQW or Cipher Hunt, you might have heard the recording from Stan Pines. They were played backwards. This is probably made of wild assumptions, but when Old Man McGucket and Gideon were possessed by Bill Cipher, they started speaking backwards briefly. Which points towards Bill being alive... in Stan's mind.
Ok I've probably gone crazy contemplating this but feel free to post your comments and ideas (and new theories, i love those)