Peach- Daisy (or shadow peach is daisy is playable)
Daisy is already a alternative costume, and I'd rather think Rosalina would get an alt costume before Shadow Peach does.
Diddy already has a color that resembles Dixie. Considering Diddy has moves involving his tail, that would mean changing the hitbox of the move to fit for Dixie's pony.
Boshi is a good choice, but Birdo isn't because of the same reasons Dixie can't be an alt. costume for Diddy: Birdo lacks the tongue and the second jump ability of Yoshi.
Already in Smash since 64...?
Zelda/Sheik- Toon Zelda tetra
Ganondorf- Toon Ganon
TOon link- Young link
Wouldn't make much sense due to the completely different body physiques.
Zero suit- Zero suit in the Bikini version
You would like that, wouldn't you...
What Demon? Besides there's already the black version. They rather should fix the black version so it doesn't drop white feathers instead of actually black ones.
Ice climbers- nana in front
Well, that does already exist, too. Some of the colour changes put Nana in front. If you mean that with the same colour, good luck in a match where 2 people take the same colour scheme except for one having Nana the other one Popo in front.
R.O.B.- Ancient Minister possibly
Ancient Minister has no arms.
Captain Falcon- Blood Falcon
Is already in as alt. colour, even with the Blood Falcon symbol on his back.
Pikachu- An oversized Pichu
Jigglypuff- An oversized Igglybuff
Although not as apparent as with Dixie and Birdo, those two wouldn't work out too well, imo.
I'm surprised you didn't put Claus as Lucas' alt (so I could tell you he's in as another alt colour scheme, the one with the redd-ish hair)... Anyway, Ninten basically looks like Ness, just with blue shoes instead red ones.
The other I can agree with, though do you mind showing me a picture of "Mrs. Game & Watch"? I've never heard of such a character.