As much as I hate to say it, I'm starting to be more accepting of the idea of Daisy.
Don't get me wrong, as a Waluigi fan, the very thought of her being included as a fighter while he's stuck as an Assist isn't a favorable outcome to me, but there's something I realized;
Literally everyone else in the typical Mario "family" is featured (in the flesh) in some way but her.
Mario, Luigi, Peach, Bowser, and Bowser Jr. are all fighters.
Yoshi, Wario, and Donkey Kong are also fighters while being representative of their own respective franchises.
Toad is Peach's meat shield and Waluigi's an Assist.
Even the more recent addition to the "family" Rosalina is given a role (as a fighter).
Hell, the Koopalings even have a place as alts for Bowser Jr. and Kamek has a role in the Super Mario Bros. U stage, in Smash Run, and in Smash Tour if you want to count them too.
So what does that leave Daisy? Closest thing is Peach having a palette that resembles her color scheme and some trophies.
With that in mind, while I'd rather have Daisy simply be an Assist like Waluigi, I'm now less opposed to her being a DLC character.