I believe it was included in the beginning of brainstorming but then it got eliminated in the process since they got to limit it into 6 stages (Space Colony Ark, Rail Canyon, Sunset Heights, Kingdom Valley, Chaos island, and Radical highway) and 4 bosses (Biolizard, Metal overlord, Mephiles, and Devil Doom). I think they are all very well picked, but let's say if you got in charge, which stage you would replace with something from Shadow(2005)?
Yet Chaos Island, from a game Shadow was never in, made the cut?
These design never showed up in any Sonic games/cartoon/comic before. I don't even know what "Luminosity" is.
https://en.wikipedia.org/wiki/Luminosity