Vaati has a chance. He's the only other reaccuring stand alone Zelda villan besides Ganondorf, and he's also the only Villan who ever got his own sword (the Four Sword) made just to defeat him aside from Ganondorf.
But, he's also only ever appeared in the cartoony style Zelda games, like Four Swords and Minish Cap (and maybe Phantom Hourglass), so I'd say that if he does get in, it'd have to be representing this style. And, at least for me, this also means that we'd need a Link in this style, because what purpose does a villan serve without the hero?
So I'm thinking that if we see Vaati in brawl, we'll also see a redone young link, in the same cartoony style, for balance.
Considering the direction the Legend of Zelda series seems to be taking, with the realistic style on the consols and the cartoony style on the handhelds simultaneously, I'd say this is fairly likely. Or at least probable.