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.
Game Rant

Every Sonic Game Worth Playing In 2025

While some of Sega's Sonic games may be a little outdated, these specific titles are still an absolute blast to play through ...