I feel the new designs fit the characters perfectly.
However, I hope that both Peach and Yoshi look colorful and vibrant; Yoshi taking his Super Mario Galaxy 2 appearance.
I am a huge fan of SS Zelda so I hope to see her design reflect SS even just a little bit! Same goes for Shiek. However, as for Ganondorf, I definitely want to see a full OoT HD outfit. That would be boss.
Much like DK and Fox, I'm REALLY looking forward to seeing what the team does with Diddy, Dixie (^_−☆), Wolf, and Falco. Hmmmmm...Falco's HD feathers....smexy!
As for design features for possible newcomers, I definitely want to see rebooted Little Mac with shiny fabric and detailed muscles, KIU Palutena with a shiny glow effect, a scaley King K. Rool (reptilian scales), a anime-esque Takamaru that combines his Samurai Warriors and his traditional image, a mix between traditional and Other M Ridz, and white-sprite Sheriff.
On a similar note, I feel it would have been awesome to have included spray effects. Basically, upon being hit by a powerful attack, certain materials eject from the smashed opponent. Examples include:
Spit from Mario and Link
Glitter from Peach and Zelda
Angel feathers from Pit
Egg shells from Yoshi
Fire from Bowser
Hair from DK and Diddy Kong
Bolts from R.O.B.
Energy emission from Samus
Discharged static electricity from Pikachu
Fur from fox and wolf
Feathers from Falco.
Etc