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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I Couldn’t Stop Laughing During The 'Sonic The Hedgehog 3' Dance Sequence, And Was Happy To Learn How Jim Carrey Made It Happen

"Sonic the Hedgehog 3" which has earned a solid review from CinemaBlend and easily sprinted past the competition at the ...