Here's what would be nice about Koopa Crew and Duck Hunt Dog:
Most of the newcomers so far have been human or humanoid: Villager, Mega Man, Wii Fit Trainer, Rosalina & Luma, Little Mac, Mii Fighters, Palutena, Lucina, Robin.
And many of the newcomers have had dark colors and/or the color blue.
The Koopa Crew and Duck Hunt Dog would add some straight-up animal mascots (distinctly Nintendo, I might add), and they would also add some color -- greens, reds, oranges, etc.
So yeah, I think they'd add nicely to the visual variety.