I'm thinking the basic Power Suit with the shoulder pads and arm cannon removed could work; the Wii Fit Trainers have slightly different body proportions but it's purely aesthetic. If they go the extra mile, they could make the removed pieces of armor look slightly damaged, to explain why they aren't present. That would also give the "Oh my, I'm not fully equipped. I suppose I'll have to handle this the hard way" feel I think they were originally trying to portray with Zero Suit Samus in Brawl.
As for Varia Suit Samus, if they simply removed her helmet as an alt. costume, they would be cool, too. It would remind me of the Prime 1 ending, as well as that rare Melee trophy. Although Varia Suit Samus has way more alt. costume potential, consider how many different suits of armor she has. Also, it would be weird if she took off her helmet and still made those robotic beeps (would be cool if she became silent, though, except for a couple of grunts). That's why I think my idea is more fitting for Zero Suit Samus.
Anyways... both versions of Samus have some of the best alt. costume and palette swap potential. Semi-off topic, but alt. costumes+palette swaps+customizable moveset = giving her armor different functions. This is already my favorite Smash Bros. Samus and I never played the game! lol