Being a Smash veteran since 64, Mario suffers from a bit of a simplistic moveset. But I think he should get the Link treatment and have his moveset updated to match the times. Competitive play will be somewhat taken into consideration.
Change 1: Replace Mario's Bair with a Tanooki Tail. This would only be cosmetic, as Mario's Bair is already a great move, but this would have much more visual flair.
Change 2: Make Mario's down air a ground pound. It boggles my mind how Mario does not have his iconic ground pound as a move yet. I know that his current Dair is extremely good for combos, so that brings me to...
Change 3: Give Mario his Down B tornado back, but keep it fast like his current Dair and give it a visual effect to reference Mario Galaxy. I like F.L.U.D.D. as a move, but it's honestly just too situational. A universally good move is better.
Change 4: Make Mario's Neutral B able to fire two fireballs simultaneously. This would match the actual move from 2D Mario games as well as make his projectile slightly better in Smash Bros.
Change 5: Fix Mario's recovery; let him glide with his cape. Mario currently has almost no recovery, and a glide would allow him to have a decent one. Not jank like Brawl, but a simple horizontal glide that can be cancelled into Up-B to give Mario an actual recovery.
Change 6: Give Mario his Smash 4 Down throw and Up tilt back. HEAR ME OUT!!! I know people complained that it was a braindead combo in Smash 4, but it wasn't broken. Mario is supposed to be the jack of all trades character that anyone can pick up. And a "braindead combo" allows for new players to understand how combos work and start playing on a more advanced level.
Plus, I want Mario to be Top Tier, so an extra guaranteed combo helps.
Change 7: Give Mario disjointed hitboxes. Unfortunately, as us Mario players know, Mario's biggest weakness is his range, or lack thereof. I'm not suggesting anything stupid like sword range, but a slight increase to his range would help Mario become top tier.
EDIT: Change 8: Make Mario's (and Luigi's) footstool jump do damage and a stronger spike. This just makes sense canonically and would give Mario a cool option.
Change 9: Mario's current Final Smash is trash, and worse, it's not even canonical! Mega Mushroom makes 1,000 times more sense.