The new special stages look good. I like the way they’ve mixed elements from the Sonic 3 & Knuckles and Sonic CD special stages together.
The Sonic 3 & Knuckles special stages now appear to be bonus stages, which is cool. I remember having a lot of fun with these, as every pre-1994 Megadrive game would unlock a different stage when locked onto the top of Sonic & Knuckles. Since I owned the Sonic & Knuckles cartridge, renting a non Sonic game would often have the added bonus of unlocking a new variation of the special stage. I guess Sonic & Knuckles used the data on the inserted cartridge to help generate the special stage’s layout and colours; procedurally generated gameplay, Megadrive style.
I think it’s more likely that we’ll see a sequel to Sonic Mania since that wouldn’t require the development of a whole new engine. If Sega release a new 2D game with updated graphics, I’d rather it be given a new name rather than being called Sonic 5. I’d rather forget that Sonic 4 exists.