Dude, I'm also interested :)
'kay, so we all know his last words, the distorted message that was actually "A-X-O-L-O-T-L! My time has come to burn! I invoke the ancient power that I may return!" backwards. This alone hints that whilst the series has ended, Bill is most likely still alive, or at the very least undefeated. Furthermore, In one of the possible storylines of Dipper and Mabel and the Curse of the Time Pirates' Treasure!: Select Your Own Choose-Venture, Dipper and Mabel end up in "the time and space between time and space," where they encounter a creature called the Axolotl —a giant talking axolotl—, who lets each of them ask him a question. Dipper asks him "what [does he] know about Bill Cipher," to which he replies with the following poem:
Sixty degrees that come in threes.
Watches from within birch trees.
Saw his own dimension burn.
Misses home and can't return.
Says he's happy. He's a liar.
Blame 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 tells us that has Axolotl probably allowed Bill to come back, even if it's in a 'different form and a different time.' The encounter with the Axolotl ends with him sending Dipper and Mabel back in time, without any memories of what happened, which means neither of them know of Bill's potential return.
Even though most of the book's content is non-canon because it follows a "choose your own adventure" format (meaning it has many possible endings), this answer is most likely the "one enormous 'canon' secret" Alex Hirsch said the book contains.
(Most of this was copied from the "Bill Cipher-Potential Return' page, so if you're confused or still want to know more, just check that out lol)
Short ver. I am alive.
What do you think?