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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Sonic the Hedgehog DC Super Heroes Crossover Bat-Shadow Action Figure Review

Sonic the Hedgehog DC Crossover Bat-Shadow Super Heroes Figures Review! Dan at Squirrel Stampede shows off the new Crossover ...