Daisy will be a playable character when Waluigi is one as well. Which means never, or at least Smash Bros 5.
One thing you all have to remember is all the characters in Smash Bros are the imporant characters to the main series they're representing. Mario, Luigi, Peach and Bowser are the most imporant character in the Mario series and are all apart of all the main games, going back to the original.
Daisy was some character that appeared at the end of Mario Land and was then forgotten for the next 10 years. Hell the only reason she was even revived is because she was the only character that would make a great partner for Peach in a Tennis game. This is why they had to create Waluigi because Wario had no parter that could exist. After that they just decided to use both character for the rest of the Mario spinoffs.
Why else does anyone think that Daisy never appears in any of the Mario Platformers. Where was she in Mario Sunshine, or New Super Mario Bros, or Mario Galaxy? Or where was Waluigi in Wario Land 4, Wario World, or all the Wario Ware games?
Until Smash Bros include minor characters which it won't do until at least the fifth installment when they run out of main characters, secondary characters like Daisy will not be playable. There's a reason the original Smash Bros in Japan was called Nintendo All-Star Dairantou Smash Brothers. Some of you guys need to FINALLY realise this because Smash Bros, has always been and always will be about the most important characters from the top franchices with a few classic characters thrown in for fun.
And before anyone say, but Wolf isn't important to Star Fox, shut up now. Since Andross is just a floating head, he can never be playable. And since Andross can't be playable, who would be second on the Star Fox villian list, Wolf. And since Wolf can be turned into a playable character he represents the main villian of the series, giving him the same important to his series that Bowser, Ganondorf and King Dedede have to theirs.