If I may make a stage suggestion:
(yes, the model is a bit scratchy)
Labyrinth Zone would be a basic stage of normal size, with it's only gimmick being water. In SSBB, when you run out of buoyancy to float on the surface of water, you simply sink to a watery grave. However, this stage has two moving rock platforms underneath the water, so upon sinking, you fall into the water but have a chance to not only save yourself, but also have underwater battles. Sometimes the water rises, leaving only the two uppermost platforms outside of the water. At random intervals, burrowbots (those badniks with drill noses) will burrow out of the ground onto the playing field to be a nuisance, as does the jaws badnik, with the Burrowbots only appearing above water and the Jaws robots only appearing below. The stage looks like it is a walk-off stage, but that's just because the image is a little too zoomed in. Past the fields of blocks on each side is a watery death pit.
Also, although the image above is a bit low res, the actual stage would be graphically appealing, with strong orangey brown stone, amazing water effects, everything having been faithfully recreated in 3D (but more like Green Hill than Mushroom Kingdom in terms of upgrades), as well as having eggman's robots from all of Sonic history slowly destroying the background terrain.
The music would be as follows:
Labyrinth Zone - Sonic 1 Megadrive
Labyrinth Zone - Sonic 1 Game Gear
Marble Zone (Remix) - Sonic 1 Megadrive
Aquatic Ruin Zone - Sonic 2 Megadrive
Hydrocity Zone - Sonic 3
Chrome Gadget - Sonic 3
Tidal Tempest Zone Present - Sonic CD USA
Tidal Tempest Zone Present - Sonic CD JP
Gallery - Sonic Jam
Lost World 1 - Sonic Adventure
Aquatic Mine - Sonic Adventure 2
So? What do you think? Would you play this stage?