It's pretty up there when it comes to quality, I haven't unlocked everything but so far the amount of choices is staggering. So yeah, it's hands down one of the best Mario games ever (and likely the best 2D platformer).
However, there are some things holding it back for me
Like the lack of more Mario art styles. Where's the art style from Super Mario RPG, or Paper Mario, or even Mario 64? (Mario 64 might be hard to re-create, but it should be possible, I'm sure they could do it if they tried)
Also, the lack of different attacks for characters of other series. I like the fact that different characters have their own personalities, but when Link picks up a fire mushroom, shouldn't he be able to throw swords like in the original LoZ?
Also, on top of that, no inclusion of Flud, maybe they could make him a power-up, and when you pick him up he says "Mario" like he would in Super Mario Sunshine.
More settings would be nice, like, maybe recreating Isle Delfino as a background, and creating your own settings, a good way to implement Mario Paint into the game!