as someone who has been fixating on all the pre-production and development and cut content of Sonic 2 since Sega was still making their own consoles, I have come to the assumption of "a lot of the cut Sonic 2 levels just got turned into Sonic 3 levels instead" Hidden Palace: well this one is blatantly obvious, although it looks like some of the decorative graphical elements in Ice Cap were inspired by ones in the prototype Hidden Palace, especially the crystalline bridges Wood/Secret Jungle: the concept art for this was altered and reused for Mushroom Hill Sand Shower: The concept for an unused desert stage in Sonic 2 likely got revisited as Sandopolis in S&K, although this time as a more Egyptian-style desert rather than the American-style one previously envisioned. Madness Mountain: An early concept art for Sonic 2 that likely served as a template for what eventually became Mystic Cave, although it seems to also have been re-used as a concept art for Act 2 of Sandopolis. Tropical Sun: A stage that never made it past very early concept art/coding, it looked like a coastal jungle that served as a 'past' counterpart to the also-cut Ocean Wind. The concept art was likely reused for designing portions of Angel Island. Blue Ocean: A stage that never made it past very early concept art. A rocky underwater stage that appeared to have a focus on navigating currents. The concept art was likely reused for designing Hydrocity. Rock World: A winter stage that served as a palette swap/"past" counterpart of Sand Shower. Its concept art was briefly seen during a behind-the-scenes TV special showing part of the production work, and it likely served as the basis for Ice Cap. Cyber City: Tom Payne's recently rediscovered art and notes for this stage show this stage had a lot of gimmicks such as electrified floors, turrets, and fans that have an 'antigravity' effect. This could have been revisited as S&K's Death Egg Death Egg original concept: The original concept for Sonic 2's Death Egg involved running on the outside surface of the vessel, which returns as a concept in Death Egg Act 2 of S&K
Fire Mountain: Although it never had any concept art or graphics made, an idea in Hirokazu Yasuhara's original design notes for Sonic 2 involved the Death Egg crashing down into a volcano which would become one of the final stages of the game. This lines up heavily with S&K's Lava Reef.
