Marth should have Fire Emblem 12/13 Marth, Sonic should have the Robo-Sim-Sonic, Link - Skyloft costume, Fox- Comand outfit(Brawl Face), Wolf- Starfox64 wolf. The possibilities are endless!
Personally I would like them to get a little more creative with it and give us characters that look astonishingly different while acting the same.
ex: Metal Sonic over Sonic (I assume you were referring to something else). He could use all sorts of mechanical SFX, change the spin moves to his Black Shield, etc. He's similar enough to Sonic to make it work without it being uninteresting. Here's the Black Shield, best known from Sonic Adventure 2 Battle:
Honestly, I'm not quite sure which direction they're gonna go with the characters after Mario, but I hope it's like the above. Replacing textures is really easy, and while it'd be nice to have things that are well rigged like Dr. Mario's coat, I would rather have things that also incorporate other differences like GFX and SFX that aren't possible without their engine (or overriding something else).
EDIT: quick changes for clarification