The reasom why we have the babies and reskinned characters in the first place is because of asset flips. Since they already have the base models and animations up to boot, they can just reuse them, change things up a bit and bam, new character like that. Literally echo fighters, bit for Mario Kart. Baby Rosalina and Pink Gold Peach would take less time than Diddy Kong or Pauline, maybe even less time than both of them combined.
Not saying that you can't hope for them, but they're just filler, no more, no less.
And before someome says put them in one slot, you just moved their clickable PNGs. The same amount of effort would still be put in each of those characters no matter where they go.