I have a few things to throw out.
First off, a level I scrapped in the extremely early stages of construction would have been a floating paradise with waterfalls falling towards the earth, seeping out from the crumbling ground below them.
A basic floating oasis if you will.
A good boss idea I came up with would have been something similar to a rocky version of the final boss from Sonic 2, where you basically hit off the head area to reveal Dr. Eggman controlling a machine of types covered with a basic rock suit.
The start of the boss could have taken place at the end of the zone and it would've been in a rock wall and eventually blasts out.
(This idea comes from the title screen of Sonic CD.
One more zone type I came up with was a "neon" styled resort stage (basically like light ups from Casino Night) with a basic montage of the tropical island themed stages such as GHZ, EHZ, AIZ and Emerald Coast, maybe even using the best enemies from all four.
Also, Overbounds moon idea made me think of one more stage I was going to do but decided not to do it.
The basis would have been inside a gigantic clock full of gear based gimmicks (never thought these out to be honest) and it would have basically had, at some point, a clock but reversed being that you would be inside.
It would have been similar to my coffee pots clock where it is blue light with black digital hands.
The point would have been to repeat the time on your PC/Mac/Dreamcast VMU/360/whatever you would use to run this game.
The reason why I bring up the moon thing is I came up with a thing while concocting my game to where it would be a way for Eggman to check the time from outer space, maybe while he's on a moon or a new Death Egg or what have you.
Kind of lame but hey.
At least it served a purpose.
I hope this is the right thing you are looking for.
If so, I have a shit ton more ideas its just that I'm tired.
Have at it.
(Mind you, I'd like to see that clock tower zone come to life one day so I hope someone definitely uses it.)