My personal opinions regarding this:
Mario: Tanooki Mario
Luigi: Bee Luigi
Peach: Shadow Queen
Bowser: Dry Bowser
Donkey Kong: DK Jr. Outfit
Diddy Kong: Super Mario Strikers Charged Uniform
Yoshi: (none)*
Wario: Overalls
Link: Skyloft Villager Outfit
Zelda: Skyward Sword Hylia Dress
Sheik: Skyward Sword Impa inspired design
Ganondorf: Ocarina of Time Ganondorf (adult era version)
Toon Link: Conductor's Uniform
Samus: Fusion Suit
Zero Suit Samus: Metroid Fusion Suitless Design
Pit: Three Sacred Treasures Armor
Ice Climbers: Climber Game & Watch inspired outfit
R.O.B.: Something based on the Ancient Minister
Kirby: (none)*
Meta Knight: Galacta Knight
King Dedede: Masked Dedede
Olimar: ???**
Fox: Star Fox Assault Design
Falco: Star Fox Adventures Design
Wolf: Star Fox 64 inspired design
Captain Falcon: Original F-Zero Design
Pikachu: (none)*
Pokemon Trainer: Female FireRed/LeafGreen Trainer (with appropriate voice change)
Charizard, Squirtle, Ivysaur: Contest Ribbons
Lucario: Riley inspired outfit
Jigglypuff: (none)*
Marth: Original Marth design
Ike: Awakening DLC Design
Ness: Pajamas
Lucas: Pigmask Disguise
Mr. Game & Watch: Nintendo Land Octopus Dance Guy
Snake: Octocamo with Mask
Sonic: Sonic Adventure 2 Battle Multiplayer Alt
I am for the most part against complete character swaps and flat out making shit up.
*: Regarding the (none)s I wrote:
Yoshi: There really isn't anything worth giving him without delving into nonsense like Mario Party 2 outfits, and I think it would better represent Yoshi to include more established Yoshi colors (like black, white, purple, etc.) so he could have six extra colors to compensate his lack of a costume.
Kirby: Any potential costume given to him needs to work with the established Kirby hats that symbolize what power he has, and honestly, nothing could really work. Extra colors for him like Yoshi.
Pikachu & Jigglypuff: They already kind of have costumes in the form of their varying hats--the best option I see for them is to give them equal amount of hats to switch through to the colors the other characters have through their two outfits.
**: Regarding Olimar, I simply don't know. I think we should wait to see what he potentially appears like in Pikmin 3 before saying anything, but that's just me.
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Another thing to consider--the transformation characters (Zelda/Sheik and Samus/ZSS) need to have their costumes correlate with each other. Also, think of how the characters do their attacks--they need to still be able to feasibly do them. Think Peach--she has a few attacks where she hits them with her dress, so that needs to be there.