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.
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Best Sonic Games That Are Not About Speed

While the Sonic the Hedgehog franchise of games may be primarily associated with fast-paced gameplay and high-speed thrills, that doesn't inherently mean that every game featuring the Blue Blur needs ...