My best explanation about having less newcomers is because of a the way the roster is organized. Notice how a new row is added in each installment (64 had 2, Melee had 3, Brawl had 4); we're expecting five rows, which why we have 5 Mario reps again. Then there's the columns, which has been 9 since Melee. Since one slot is being used for the random button from Brawl, which hasn't been confirmed for return yet, I'm expecting about 9 characters to fill in those spots. So far 5 newcomers are confirmed; that means we only have 4 slots left for the newcomers + Mewtwo.