I think uniqueness plays a large role for one basic reason; there are no absolutely essential characters left. Brawl mostly filled in tiny areas, like rounding out Kirby and adding Wario and such, and the rest of the characters were sort of bonuses. Nearly every series has its vital characters that the franchise relies on (the few sort-of exceptions being Ridley and possibly K. Rool to round out their respective villain roles)
Since nearly every big Nintendo franchise is getting the representation it deserves, now all that's really left is to make the game as fun as possible, which means giving us loads of variety in terms of new characters and such. Rosalina may not really be as important as Toad or Bowser Jr., but she was one of the most unique choices out there. Wii Fit isn't a fighting franchise, but Nintendo really doesn't have anything else in its arsenal like it, meaning the Wii Fit Trainer can bring abilities nobody else can. At the end of the day, the game needs to be fun, and the best way to do that is to bring us characters from smaller franchises with abilities nobody else has, and smaller interesting characters from the big franchises.
Of course, this makes Fire Emblem confusing as hell because all of Fire Emblem's interesting characters are the unimportant ones :/