I don't think Daisy would be included anytime soon. it would be cool if she was , along with Waluigi , to have a full Mario cast.
Then again , what would Daisy do or represent that Peach doesn't
First of all , Daisy has no characteristics on her own besides using flowers , and those abilities are even sometimes reduced to being a copy of Peach signature hearts (see Mario Super Star Baseball)
In her debut Daisy did nothing , theres nothing that could be taken from there either.
Meanwhile another signature of Peach , that appears even outside Smash , is her Floating ability from SMB2.
Peach already has references to Golf And Tennis as she includes them in her side Smash , along with Super Mario RPG frying pan.
Yet another trademark of Peach is her umbrella. She uses it too.
Point is , many quirks that the characters have are unique to them from other games , nobody else does them
As far as i know , Daisy does nothing that Peach doesn't , but Peach indeed does things Daisy doesn't.
While Sakurai is perfectly capable to come out with something for Daisy , if her inclusion is planned , she as a character doesn't really have much going on for her unlike Peach or even Rosalina , who floats , has her wand , her magic powers and affiliation with Lumas.
Lastly whatever related to spin off , it seems to be also associated with Waluigi , as he was picked to be the character holding the Mario Tennis racket as an assist Trophy and not Daisy.