Somewhere at Sonic Team is a whole "roadmap of Sonic titles," says head honcho Takashi Iizuka, and the devs "think about which game to release in which year"

Sonic Team head Takashi Iizuka has explained how the studio plans out the releases of Sonic games on a roadmap internally.
Kotaku

Sonic Adventure Advanced

All the Latest Game Footage and Images from Sonic Adventure Advanced Sonic Adventure Advanced is a 2D recreation of Sonic Adventure, aimed to translate the original game into 2D, while also including ...