Final Fever was a newly designed level that Christian "The Taxman" Whitehead planned to add to his officially licensed 2011 remake of Sonic CD but ended up cancelling. It was intended to be a boss only for players who had collected all the Time Stones (i.e. encountered in the Good Future only).
Time was to flow freely, and Dr. Eggman's machine would be almost invincible: the idea was that Sonic could reach warp speed at any opportunity (instead of having to pass a warp post) and must do so in order to "wind back the clock" and thereby damage Eggman. Some parts are running, whereas in some areas the screen is locked.
The level was to use the Final Fever boss music track from Metallic Madness 3. The Metallic Madness 3 boss would then use the normal boss music if all the Time Stones were collected.