Sonic Saturn
Main game
Alongside the production of Sonic X-treme, STI was working on another game only known as "Sonic Saturn". Rather than use sprites for sonic, though, it was going to use a 3D model for Sonic. This game was meant to have a more realistic rendition of Sonic's world than X-treme. It was never actually announced publicly that it was in production, but didn't get very far before being canceled.