You can also download a Big Red Button internal PDF of February 2014 here, it's a design guide in which you can see various concepts and ideas for the game and how it evolved, including several things discarded.
You can read a translation of the article below, courtesy of Fusion Ellipis from SSMB. Spanish site Sonic Paradise published a whole slew of new information about the game's development thanks to the efforts of researcher MotoBadnik, who was not only able to uncover some prototype trailers of the game from 20 (back when it was known as 'Sonic Synergy'), but also secured an interview with a former Big Red Button employee (who requested anonymity). Here's one hell of a whopper update on the game's development history.
