Now, there's some possibilities. See, in the original un-cut Sonic CD ending there is a stage just between PPanic and CC.

Now in a video revealing some of Sonic CD's development we saw a sketch of Dubious Depths (r2) revealed.

This image is most likely used for the Good Future, and it matches with the Little Planet theme song which, surprise, was ACTUALLY the PRESENT theme for r2. (Now I just edited this. I thought that the Little Planet theme WAS THE GOOD FUTURE MUSIC for r2, but it turns out it's the present theme, that's why this part is kind of not making sense sort of. Well, back to the reading!)

Here we can see what is most-likely art for r2. Now we can see the "Depths" of r2. As you can see how it goes inside the floor, like Marble Zone.

Before I continue I just wanna say that the theory of Sonic CD's stages matching up with Sonic 1 may be correct.

See, in Sonic 1 we have the all iconic 6 zones (excluding "Final Zone")

Green Hill

MARBLE

Spring Yard

Labyrinth

Star Light

Scrap Brain

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Now you may have noticed I have Marble Zone in CAPS, I've done that for a reason.

See, in Sonic CD we have 6 zones (there is no "Final Zone" like in Sonic 1)

Palmtree Panic

Collision Chaos

Tidal Tempest

Quartz Quadrant

Wacky Workbench

Stardust Speedway

Metallic Madness

These zones match up with the ones in Sonic 1 which I will show now.

Palmtree Panic = Green Hill

Collision Chaos = Spring Yard

Tidal Tempest = Labyrinth Zone

Quartz Quadrant = (no match with a zone in Sonic 1)

Wacky Workbench = (no match with a zone in Sonic 1)

Stardust Speedway = Star Light

Metallic Madness = Scrap Brain

Besides QQ and WW, the zones match up! But no zone matched up with Marble Zone. See, if Dubious Depths was still in the game then Marble will match up with that.

Anyway. In the sketch of r2 with the rainbow fountain is most likely the good future of r2, considering it looks happy.

As we stated earlier, Little Planet's theme in the D.A Garden is r2's future theme. In a Sonic album, Sonic Techno mix at timestamp 3:19 we can hear the bad future theme of DD. https://www.youtube.com/watch?v=tuvv0jFysk4. Now I am also 98% sure I also heard DD bad future at the very start at the album as well. Here is also a cut from the same album. https://www.youtube.com/watch?v=yfKeO69tKgI. At 0:31 in DA Garden/Little Planet's Theme we can also hear the same motif in the DD bad future from the album https://www.youtube.com/watch?v=gOKVHnoW018.

Well, that's all for now, hopefully this was a fun read. See you.