Yesterday, E3 gameplay trailers were released for both Sonic Boom: Rise of Lyric and Sonic Boom: Shattered Crystal. No one noticed a small detail, however, until today, when a Sonic Stadium member pointed it out at the 34-second mark.
To the left of the screen shown, a figure identical to Metal Sonic is clearly visible. Sega hasn't officially confirmed him, but this is good enough for me. ;)