Early in the game the character "Sackboy" from LittleBigPlanet makes a cameo as a clue.
During the 2014 holiday season a trailer was released using the song silent night
Early in the game there is a sign for Kratos Cycles with the caption "Ride like a god." This possibly alludes to Kratos in Sony's God of War series who is the illegitimate son of Zeus.
A video containing a full playthrough of the single-player campaign was leaked a week prior to the official release.