To put it more accurately, we're not getting anyone else other than Mewtwo, Lucas, Roy, Ryu, and 2 ballot characters, and makes sense. Here's what the roster currently looks with Mewtwo:
We can assume that that empty space in the character select screen is for Lucas, and notice the number of Zelda characters in the 2nd row while Link is at the 1st row. Once Roy, Ryu, and the two ballot rosters are added into the roster, the Zelda characters will all be on the 1st row. Attempting to add more characters outside of the 6 characters would only disorganize the roster again and mess with everyone's OCD.