Countering Daisy's issues huh?
"Daisy only appears in spin-offs. Rosalina appeared in a main game. Thus she is a main Mario character and relevant"
While Daisy has not appeared in many main Mario games, this doesn't mean she's not a main Mario character.
She appeared in almost every spin-off ever. And when counting these, appearing in most of them is pretty major in and of itself.
The Mario spin-offs will always continue time and time again, and they will all have Daisy in the roster.
Not only does she appear in most of the spin-offs, she also appears in one of the first, making her have senority while being relevant.
Also, there's no official rule that says the spin-offs are inferior to the least major main games.
That's just a fan-rule.
Fact is, Daisy appears more than the rest of the potential candidates, from 90's to now, and has no reason to stop doing that.
Btw, this character probably appeared in more games than your whole top 5 appeared together.
Even King K. Rool dwarfs in comparison to the Flower Princess.
"Daisy has no moveset potential/no clear archetype/is a Peach-clone. Rosalina had spacey stuff and Lumas to work with."
Just like Rosalina was just a Space Peach, amirite
Daisy has a lot of moveset potential for the previous answer given.
Most spin-offs involve special moves and catergories (All-Round, Speed etc.)
Thus we have a very good sense of Daisy's potential and archetype when looking at these.
Looking at it, it seems Daisy has an affinity for Flowers and is a speedy and technical character.
Now there's something important to say here:
The moveset is not made by solely taping together the individual special moves.
There are creative liberties to take when making these.
While a basketball dunk is the only recovery move that is a spin-off move, a better fit would be a Petal Tornado or a Flower Glider (Mario Kart 8 or Super Mario Galaxy 2)
Another example is a garden special that, while appearing in Mario Power Tennis, could house different plant-like enemies to damage enemies, or could release pollen, petals and other plant-stuff to hurt the opponent.
Last, but not least, there are a lot of Mario plantpowers that Daisy could use.
There are dozens of flowers that give actual powers, like Fire Flower.
A lot of plant-like enemies, like Nippers.
And there are a lot of flower-based hazards that could be planted over to Daisy's movekit. (Piranha Vines, Petey Piranha)
See, there's a lot to do with both real-life plant physics (pollen, petals) , Mario's many plant enemies (Piranha Plant, Nipper) and Mario's flower powers (Fire Flower, Flower Glider)
"No new Mario character, it's impossible. Rosalina and the Big Bowser's Kid are enough. Daisy's unneeded."
Just as unneeded as having Bowser Jr. AND Rosalina in the base game, right?
The Mario series sell extremely high, being the most selling franchise from Nintendo.
DLC needs to sell on his own, and what's a better solution than selling a Mario character?
As for "reps", Sakurai added two completely unique Mario characters in the base game already.
No stopping him from adding the third.
"She's so filler"
Um, yeah, that's why Daisy appears even on a roster of 10.
She might be filler, but she's a pretty good one as she fills the Mario Spin-Off roster every single time.
BTW, This is a non-argument, everybody could be said to be filler as every character is made to fill a specific role.
Heck, even Mario is a filler character made to fill-in the lack of Popeye in the original.
Ex.
"Um yeah King K. Rool is so filler they just needed a villain"
"Mario is so filler they just needed a jumpguy after popeye didn't do it"
"Luigi so filler, he's only player two"
"Ridley so filler, they just needed more bosses"
"Daisy has no personality, I, Rosalina, am at least always super sad with echoes."
Being always happy and girly and rainbows (Peach), is as much as a personality as a loud and dense tomboy (Daisy)