Well I don't have many clues for this but there are some intresting facts. Stan finds a pair of glasses on Carpet Diem, but when the Author comes back he wears his. Also there are three journals one for each brother. And in Bill Cipher's ciricle there are ten symbols (Tree-Dipper, Shooting Star-Mable, Question Mark-Soos, Symbol from Stan's fez-Stan, Ice-Wendy. Llama-Pacifica, Star with the Eye-Giddeon, Heart-Robbie) and there are two left the Hand with the Six FIngers and the Glasses. If hand belongs to the Autor then the glasses are avaliable. Th last brother might be a bad guy or the guy that died in the car accident (and that leaves the last journal to Dipper). And finaly one of them mustn't be Dippers and Mable's grandfather???