So yes, uniqueness is king this time around, though we do see that popularity is still a factor too.
Well, let's take a look at what we have so far and what we can see in the future...
Villager: A character with a unique moveset of using everything he can get his hands on and staying true to the loving and carefree nature of the Animal Crossing series.
MegaMan: A character who is the first to wield projectile Smash and normal attacks as well as wielding a moveset full of Robot Master powers.
Wii Fit Trainer: A character who fights using different exercises and poses, almost along the lines of accidental fighting.
Rosalina & Luma: A character that uses a somewhat-secondary character in a majority of her attacks, but instead of just being luggage like Nana, the Luma has a serious contribution to everything you do, making Rosalina the ultimate set-up character.
What we could see to fit the uniqueness bill in the future:
Ridley: A character that uses a fighting style that connects ground-based attacks into aerial attacks and hovering/flying abilities, then following up with several different attacks to string combos together. Also utilizes projectiles and tail attacks to continue attacking options.
Shulk: A sword wielder who can use a level-changing system, whose attacks grow stronger but limits range as the level increases. Levels increase with damage done to opponents and resets after every KO.
King K.Rool: A loony heavyweight character that is surprisingly faster and more mobile than that of other heavyweights, throwing around his weight to deal heavy damage in his own insane ways, including the use of a Blunderbuss, Boxing Gloves and more. The drawback is that combo potential is relatively low with this character.
Little Mac: A character who is built around counters, just like the initial Punch-Out titles. All attacks focusing on punches, this character also wields a power-up system for its attacks in the form of the Star System, which increases once this character performs certain attacks, performing a Forward Smash will cost Stars, increasing the damage of the attack. A few other attack options include stun attacks and projectile counters.
Mewtwo: First off, MASSIVE popularity. Secondly, this character utilizes a powerful yet semi-floaty style that's meant to hit hard and deal damage from all ranges. The overall combo potential is lowered, but the damage output is MUCH higher than Melee days. Not to mention the inclusion of new attacks like Psycho Cut and Skill Swap.