Sonic games are full of characters that have existed in the form of both a major and only one off appearance, many of the past characters introduced in the Sonic franchise have not even really had a place in the more recent Sonic games, the only real staples of the Sonic franchise include Sonic himself as well as Tails, Knuckles and Amy. With so many characters left unused for so long Sega did go somewhere to give many of the past characters even a slight nod.
In Sonic Unleashed many of the street levels have actually been named after characters that don't appear in the game. More specifically the members of Team Dark (Shadow, Rouge and E-123 Omega), Silver, Blaze, Pachacamac, and the Chao. Exceptions to this include Knuckles and Cream. This at least has allowed the developers to pay homage to many of the characters used in the past even if they haven't made an appearance in a while. I reckon future games should take this advice and make use of these extra characters, even if they only come in name, we already know that Sonic Boom really only has about four of the original characters playable but hopefully we might see more.