Hang on. Let me get my notepad...
All the Mario cast > Mario Strikers outfits, for those that have it available, Biker outfits for the girls
All the N64 cast > N64 models/Fighting Polygon alts
All Pokémon > Shiny variants
> Plenty to choose from out of Odyssey alone, but Sunshine outfit probably stands out most
> Dr. Luigi, Mr. L, original Luigi variant of the Wedding/Builder outfits
> Plenty to choose from out of Odyssey alone, Wedding outfit in particular, Delfino Island outfit, and Classic outfit
> Wedding outfit
> Scientist outfit from Odyssey
> Fire Rosalina
> Sunshine Mask/Bandana
> Woolly Yoshi (different from Crafter World's Plush Yoshi), Boshi, and
all the Yoshi colors
> Boxing Gloves
> Sunglasses + backwards cap
> Ocarina of Time, Twilight Princess, Hyrule Warriors outfits
> Ocarina of Time, Twilight Princess, Hyrule Warriors outfits
> Ocarina of Time, ''Twilight Princess'', Hyrule Warriors outfits
> Twilight Princess, Hyrule Warriors outfits
> Majora's Mask outfit, maybe take inspiration from Hyrule Warriors and place a mask on him (not necessarily on his face, but maybe a taunt could allow you to put it on or off his face)
> Conductor's outfit, pajamas
> Metroid Prime varia suit
> Loose hair, no ponytail
> More colors, copy ability doesn't really allow for proper outfits I don't think
> Maskless
> Proper masked costume
> Melee outfit
> Melee outfit
> Brawl outfit
> More female trainer caps for female Pikachu alt
> Holding microphone
> Spiky-ear
> Movie armor outfit
> Other trainers, maybe Gary/Blue
>>> Squirtle, Ivysaur, Charizard > Have the option to make them wear the same cap as their respective trainers, could allow you to mix and match colors for each while still being able to tell which is yours when the same color is used among trainers
> Sir Aaron's hat
> Ash Greninja
> Classic SNES outfit
> Pajamas
> Pajamas
> Outfit based on the polar bears from their home game (polar bear parka)
> Melee, Brawl outfit
> Melee outfit
> Awakening DLC outfit
:ultrobin > Hooded
> Masked
> On a stick
> More colors
> Classic looks
> Classic looks
> Classic variant based on Pit's classic looks
> Any of his 'Master of Disguise' outfits
> Possibly more of the Pikmin 3 protagonists?
> Virtual Boy head
> More faces/hairstyles (the latter in particular), maybe add hats for some
> Classic looks
> Wii Fit U Trainer
> Actual alternate outfits from Xenoblade that don't strip him down to his underwear
> More dog types
> Other Snake appearances (Big Boss, Naked Snake, whatever else there is)
> Anything that isn't blue for a change
> Helmetless
> Anything that isn't yellow for a change
> Wearing shoes like he does in Street Fighter 1, along with that game's hairstyle
< Electric fan
> Eventual Bayonetta 3 outfit, maybe a proper Jeanne alternate
> Stop hogging all the outfit spotlights
> Classic outfit, maybe some similar outfits to what Peach wears in Odyssey, but stylized for Daisy
> Boxing outfit
> More colors for his Meta-Ridley alt?
> SA-X
> Tokyo Mirage Sessions outfit
> Splatoon 2 hairstyles and outfits
> Digby alternate
> Simon's Quest looks
> Symphony of the Night looks
So, so many possibilities, I really hope we're going to get lots of good alternate costumes in this game. But I already have my Plush Yoshi, so I'm happy even if nothing from my list above is added.