I think that, for variety sake, a non humanoid character could have its chance, for the last slot.
Well, to not break too much the leitmotiv of this pass let's do that is not a human but still something near to it, like for example, mmm, a primate.
Then, since Smash has still a roster severely unbalanced toward male reps, let's choose a female one.
And mmm, since Nintendo seems to like this color for their characters hair, let's say she will have blond and long hair.
Oh, and since there is the rule we can't end on a megaton of hype character, we should choose a first party character, maybe from an already represented series, which still can provide many contents in terms of songs and spirits table, and moreover a series that is still active and it's very likely will be a new game next year, so it can also be an advertising/shilling character, like all revealed-at-the-end-of-a-dlc-cycle character.
Ok, but WHO should we pick? Mmm, some ideas anyone?