Here's my hand at a stage. Not great, but it's something:
I have an idea based on Dr. Mario. I think the sub-series is due for a stage. This would be the home stage of Dr. Mario (who would be Mario's alternate costume in SSB4).
Dr. Mario Rx
The stage would look like a vitamin capsule but much larger and takes up 2/3 of the screen in width (capsule height is roughly the same), so the main focus of KOing opponent is to emphasize on vertical attacks over horizontal ones. It's like a plain stage, but thirty seconds in, a red, blue, and yellow virus appears to wreck havoc on the players. If you touch a blue virus, you will get frozen, a red virus will burn you, and a yellow virus will shock you. A few seconds later, the mega-vitamins will arrive to kill the virus. If you end up being hit by the mega-vitamins, you will be sent up and if you are in an areas where the bottleneck hole is exposed, depending on if you have a high % of damage, you will likely be KO'd, so it's vital to avoid the mega-vitamins. Once all three viruses are killed, they will not re-spawn for thirty-seconds.
Here's some examples of music that could be in:
Title (Dr. Mario Online Rx)
Dr. Mario (Super Smash Bros. Melee)
Chill (Super Smash Bros Brawl)
Cough (Dr. Mario Online Rx)
Sneeze (Dr. Mario Online Rx)
Tetris Type A (Super Smash Bros. Brawl)
Tetris Type: B (Super Smash Bros. Brawl
Lip's Theme (Super Smash Bros. Brawl)
Pure's Theme (Panel De Pon)
Not twelve, but they are some examples