There is a lot of rumor and speculation that a new DLC update will be available for Sonic and All-Stars Racing Transformed this Wednesday. A Steam database entry earlier this month and forum posts this week are possibly confirming this.
via N4G: pc news feed http://n4g.com/news/1395655/is-sonic-and-all-stars-racing-transformed-dlc-coming
via N4G: pc news feed http://n4g.com/news/1395655/is-sonic-and-all-stars-racing-transformed-dlc-coming