Disowned Content: Rapper Mario's Stage
From the JPN Super Mario Pinball commercial. You fight where Mario's standing in the ad, and if hazards are on, then giant mechanical semi-spheres will occasionallly slam into each other on random parts of the stage, potentially insta-KO'ing you.
I'm sad that DK Rap Studios lost the DK stage vote, so here's another minimalist and meme-worthy submission. Unlike that stage, this one would have multiple music tracks, preferably all Mario rap songs like the SMBSS theme and Practice Makes Perfect.
Cappy Move: Cap Toss (Netural B)
I know it's a bit controversial to get rid of the iconic Fireball, but hopefully I can make my reasoning clear.
Mario tosses his cap forward, Cappy's eyes appearing as he does so. Cappy will stay in place at the length of a BF platform from Mario for two seconds before returning. If Cappy touches an opponent at any time between being thrown and coming back to Mario, Mario will instantly snap him back on in a quick motion, like how Terry snaps back his cap in Fatal Fury. The toss is much faster than Fireballs, but the long hitbox makes up for that. Mario touching the hat while its hovering in place will just lead to him putting it back on immediately. Hitting an opponent has the same effect as them being hit by a Fireball.
Speaking of which, ever since I was a kid I was annoyed that Mario could shoot fireballs in Smash without a Fire Flower (ignoring his Fire Mario alt). I'm not as bothered by it anymore but it still feels odd. Also, he already has his forward smash which is a pretty iconic move at this point and can be representative of Mario's fire. That is all.