It sounds like Sonic’s Ultimate Genesis Collection was not enough for Sega, because achievements have been found for three of their greatest titles that most folks may have forgotten. The folks over at MyGamerCard are to be thanked for the discovery of these achievements.
It looks as if Altered Beast, Shinobi, and Comix Zone has all but been officially confirmed by Sega themselves. Each of the achievement listings found below are legitimate. Sounds like a second round of classic Sega games will be coming to Xbox Live Arcade sometime in the future.
Source for Altered Beast: Achievement List
Source for Shinobi: Achievement List
Source for Comix Zone: Achievement List