Overalls are part of Mario's style. You can't ditch that anymore.
That said, I'd like to see a return to the Smash Bros. Melee Mario, where Mario was actually thinner for once. Same with Bowser. Those two are getting a bit too round and stale in their latest incarnations, you just don't see them change appearance much anymore. The Smash Bros. Melee versions had some subtle variations that went a long way.
Actually, I wish all the characters in Smash Bros. Melee stayed with that look for their later respective games. I look at the Smash Bros. Melee version of Falco, I look at all the other incarnations of Falco, and just marvel at the huge difference in quality. Falco has and may always look his best in SSBM. Unless Smash Bros. for Revolution rounds him out even more.