According to an IGN Article, there will be more hidden games in Sonic Gems Colection:
“Sources in Japan are reporting that, rather than just one entry in the Streets of Rage series, Sega will be including all three entries as part of the collection. Streets of Rage apparently won’t be alone. Sega will also be including the original caper game, Bonanza Brothers with the collection, our sources report. And this, apparently, isn’t the end of the fun in this fun-filled collection.”
Could this mean the inclusion of other missing games, namely Segasonic and Chaotix?