Naturally Nintendo needs her to be damsel so they can continue to have unpopular Yellow and Blue Toad.
Is it really so hard to just use Wario and Waluigi?
She could be unlockable (as Luigi is in several titles), and/or they could feature Daisy and Rosalina as stand-ins.
I agree that Wario and Waluigi would be good choices, as they would emphasize the not-so-trustworthy nature of the multiplayer interaction (even if they are villains they do butt heads with Bowser once in a while; heck Wario was in the SM64 remake). Waluigi might be a little tricky due to his body size, but a little hunch to his walk would probably work well enough.
And I've always been kinda miffed at the whole multi-colored Toad thing to being with, since they could have just used "the" properly colored Toad and Toadette instead of the blue/yellow clones.
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