What I think...Old Man Mcgucket wrote Dipper's journal or all of the journals. Mcgucket ended up crazy because of Bill Cipher entering his mind and drove Mcgucket crazy. There is also blood on Dipper's Bill Cipher page. Bill probably cut off one of Mcgucket's fingers while he was writing the book. Plus he's old timey and has had a lot of experience in Gravity Falls. If its not him, my 2nd guess is manly Dan because Wendy was born with FOUR FINGERS.
Next, I think Candy has a crush on Dipper. Why? When the Summerween Trickster appears somewhere in the Summerween episode, Candy holds on to Dipper like most romantic stuff.
Next up, I think Soos is Bigfoot in the theme song. Same size, right build, it HAS to be Soos.He is probably playing stupid during work and trying to put an end to the Mystery Shack by dressingup as Bigfoot just to work somewhere else. This is a crazy theory so I bet it won't be true
Post more theories in the comments!