So, on the topic of newcomers, here are some character I would find nice:
Just like in the original NES game, this character ought to be quite slow but powerful.
Could potentially work as dual character (Ice Climbers) if his opponent is also used.
Seems like a rather natural choice, though for licensing issues it may not be.
Anyway, I suppose he could have a spinach meter which increased his power there more he eats.
I really like the Electroplankton event in Brawl, and they can obviously be implemented in some way.
Perhaps have it some kind of floating/flying character and you can change between different planktons.
Amiibo $$$. Not sure what the playstyle would be, but certainly something wacky.
Don't even wanna consider the Final Smash.
Pac-Man is already in, so why not include Don-chan as well?
Melee character, could be centered around rhythm in some way.
I find Joy Mech Fight quite incredible for being a NES game. And as it is one of Nintendo's earliest fighting games, I think he's a given.
Sadly the game never saw a release in the west. Was included as a Sticker in Brawl though.
Zip as a character may not be too interesting, but the way you could control him could be. Imagine that you don't use the analog stick, the D-pad or any other conventional way of moving.
Instead you use the gyroscope to fling him around the stage; attacks are carried out via the A-Y buttons, shoulder buttons etc.