Sonic Saturn (cancelled Sega Saturn prototype of game; mid-1990s)
During the production of Sonic X-Treme, Sega Technical Institute were planning to make another Sonic game for Sega's ill-fated console, the Sega Saturn. The game's work working title was Sonic Saturn.
Unlike Sonic X-Treme, the game's world was meant to be more realistic. Based on what very little concept art that has actually been leaked, it (like X-Treme) appeared to have Metal Sonic in it.
It was never actually announced to the general public, and also didn't get far into production before cancellation.
Like other Sonic the Hedgehog games, Sonic Saturn was planned to have 3D special stages. (Similar to Sonic 2, Sonic 3 and Knuckles, Sonic 3D Blast, e.t.c.) It had Sonic in a 3D environment playing pool. The objective (like other main-series Sonic games) of the special stages are assumed to be to collect the seven Chaos Emeralds.