Like the Wanted Poster bearing the cameos of several characters, there are also two Missing Posters of Mighty the Armadillo from Knuckles' Chaotix (1995) and Ray the Flying Squirrel from SegaSonic the Hedgehog (1993). Both posters say "Missing since 1993."
Shadow gives over the fourth Chaos Emerald in this game, a possible tribute to Shadow's memetic line, "Where's that damn fourth Chaos Emerald?" from his earlier spin off title, Shadow the Hedgehog (2005).
Sega's associate brand manager, Aaron Webber (II), stated that Sega wanted to get Jaleel White (who voiced Sonic in The Adventures of Sonic the Hedgehog, Sonic the Hedgehog, and Sonic Underground) to provide the voice for classic sonic. However, Sega was unable to get White to reprise the role.
Several lesser known and abandoned characters from the Sonic franchise make cameos on posters in the City Escape Stage: Fang the Sniper (aka Nack the Weasel), first introduced in Sonic the Hedgehog: Triple Trouble (1994) on the Sega Game gear and made his last official appearance in Sonic: The Fighters (1996). He also appeared on one piece of fan art that made it into Sonic and the Black Knight (2009). Bean the Dynamite and Bark the Polar Bear who were also in Sonic the Fighters but never seen in any Sonic game since.
Crisis City was erased from the timeline after the events of Sonic '06. This means that Sonic should not remember that level at all, and that the level should not even exist anyway, as that future was prevented from occurring.
Early on in development, a third Sonic character representing the Dreamcast Era would have been in the game. Ryan Drummond the voice actor for Sonic in Sonic Adventure 2 (2001) and Sonic Heroes (2003) was invited to return for this role but Drummond refused as it would have required him to leave his union.