I think we will be getting 3 returning vets and 3 newcomers.
Vets include Mewtwo, Lucas, and Wolf.
My list for newcomers?
1st is King K. Rool, as he would be a unique heavy and give the DK series a much needed rep.
2nd is Ray MK I from the Custom Robo Series. His moveset would work beautifully in Smash, with guns, bombs, pods, and dash attacks rounding his moveset.
3rd is....I don't really care.
If they planned on adding another 3rd party rep, I would choose Simon Belmont from Castlevania. He is a retro character that could stand alongside Mega Man, Pac Man, and Sonic. His moveset is also not difficult to figure out with holy water, axes, and his trusty whip.
Paper Mario would be a unique Mario rep representing the RPG series using his partners, hammer and paper moves in battle.
If they wanted to be awesome, they could add ExciteBiker, another old school Nintendo Rep. He was actually briefly considered for Melee, but was replaced with the Ice Climbers instead.