So, I totally think the leak's true, though I'm not sold on those characters being the only newcomers. However letting aside the chance of having more unexpected characters, I'd like to analyze the ones we're almost sure of:
Pac-Man - 1980
Little Mac - 1984
Palutena - 1986
Mega Man - 1987
Villager - 2001
Mii - 2006
Rosalina - 2007
Wii Fit Trainer - 2007
Chorus Kids - 2008
Shulk - 2010
Chrom - 2012
Greninja - 2013
Now, notice something? Yeah, the presence of recently born characters overwhelms that of old and enstablished ones; also we're getting either very old characters or very recent ones, the nineties are a gaping hole. Now let's look at the 4 vintage characters:
Pac-Man - 1980
Little Mac - 1984
Palutena - 1986
Mega Man - 1987
Two are third party, so they don't really count as Nintendo characters. The other two have seen their series brought to life after a very, very long time, so they still seem to be chosen due to chronological relevance. That's especially true in Palutena's case: she may be from a very old series, but from what I gathered she appeared in the original game very seldom and had no personality, yet she became crucial in Uprising.
I mean, look at her back then.
So one could make an argument that the only real old-school Nintendo character they added is Little Mac. That's kind of like the opposite of what all the other entries in the series so far did. Ness and Samus weren't exactly at the peak of their popularity when Smash 64 came out. Melee had Ice Climbers, Mr.GW, while adding many characters that were staples of their respective series, such as Marth, Ganondorf, Peach, Bowser, Zelda. Brawl went all out with old, recurring and very recognizable charcaters: Diddy, Dedede, Meta Knight, Pit, Pokémon Trainer, Wario and ROB.
Yet in SSB4 almost everyone has been created in the last decade or so. Weird.
That's why I think that a few more hidden gems from the past are in the order of things.