Cancelled in 1996, the greatest 'What if?' in gaming history. It would be a few more years before the world experienced the first fully 3D Sonic title.
@AndrewC70136680@SonicSimulator you can see a tiny bit of the new "worlds" and on the left it says « Emerald Coast Stage » which means its… well… a stage not a world, therefore it (probably) won’t be locked behind rebirths. The whole thing appears to be in City Escape which needs 3 rebirths to unlock.