|Saw nothing of this on TCRF, so I figured I'd do a bit of infodumping in here. Dunno if anyone else has found any of these things, but here goes.
These are leftovers from the original stage select in the arcade version of the game, seen here. Super Monkey Ball instead uses sprites for them.
On the other hand ... this is a lot more interesting. The arcade game never had a selectable Master difficulty, and in fact it only had one single master floor altogether. Super Monkey Ball added nine other Master floors, but yet you still couldn't start straight at the Master stages from the difficulty select; SMB2 was the first to allow you to do that. Whether they were planning on making Master an actual difficulty setting in the original arcade game, or if this is something they were planning on expanding on much more than they actually did in the process of bringing the game to the GC is hard to tell. 100 floors seems a bit absurd, though...
Interestingly, the announcer sound clip that plays when selecting Master difficulty in SMB2 exists in the original as well.
Speaking of leftover things, and Master difficulty, the original Master floor's design remains intact. It's using the wrong theme, and lacks the 'sparkling floor' effect the arcade game had, however. I'd wish they'd kept that effect, but I suppose it wouldn't have looked so great in the much, much brighter theme they made for the Master stages.
Another straight port from the arcade game before it got touchups. The circle the AV logo spins on is clearly visible (it's invisible in final versions of the floor), and the logo itself lacks the reflections that make it somewhat difficult to see where the floor ends.
There's a few other stages lying around that were taken straight from the arcade game with barely any/no touchups, but I haven't bothered taking screenshots of them (Dolphin does not play well with the stage select menu for whatever reason).
On the other hand, this sole floor is never seen at all in the game! Three slanted rings, the middle one twice as large as the others. There's also arrow decals just floating in the middle of the air, and the floor is bumpy all over.
A little closer view to show the curvature of the floor, and so you can see just how bumpy that floor really is.
And a few other random tidbits, looking around the theme models...
There's only one warp goal in the floors that use the Sunset theme (Advanced 6), and it's a green warp. This goal is never seen.
Similarly, none of the floors using the Ice theme even have warps in the first place, leaving both of these random warp goals completely unused.
This is interesting, given that the Expert stages are identical to the arcade game's (which didn't have separate goals per theme, but just a general design colored blue/green/red for all of them). Why create warp goals for a theme when you know they'll never be used? Indeed, the desert theme, which also completely lacks warps, doesn't contain any green/red colored goals... maybe they had something else planned?
(Interestingly, there actually is a green warp in a Desert-themed level in SMBDX. It looked like they just took the original goal for that theme and took a paint tool to it, if I recall correctly.)
There's definitely more than this still hanging around, I just need to stop being lazy and take pictures. =p