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.
Comic Book Resources on MSN

Sonic Team Head Reveals Why Sonic the Hedgehog Games Aren't Getting Remakes

Sonic the Hedgehog is running firmly towards the future, and the studio head has now revealed why he won't be looking back.
Hosted on MSN

Sonic the Hedgehog's Takashi Iizuka says a remake of Sonic Adventure isn't in the works because Sega developers "could make a brand new game instead"

Remakes and remasters and re-ups are all the rage right now, but Sonic Team boss Takashi Iizuka has said the developer isn't too interested in returning to the Blue Blur's early 3D adventures, like ...