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.
IGN

Keanu Reeves Will Headline an Upcoming Sonic x Shadow Generations DLC - State of Play

We already knew Keanu Reeves would be voicing Shadow in the upcoming Sonic the Hedgehog 3, but he's taking his voice talents to Sega's video games as well. Announced during today's Sony State of Play ...