Based on interviews, the timeline, and what's the roster currently looks like and what we know who is guaranteed to appear in Soul Calibur 6. I present, the final roster:
Returning Revealed:
Maxi
Xianghua
Kilik
Ivy
Yoshimitsu
Sophitia
Taki
Zasalamel
Nightmare
Siegfried
Mitsurugi
Newcomers:
Groh
Geralt
Unrevealed:
Talim
Astaroth
Rock
Hwang
Seong Mina
Voldo
Cervantes
Lizardman
Edgemaster
*New Character*
And that's it. Gotta remember, we have 13 characters already revealed. And 22-23 slots left. So that leaves only ten slots, two of which we know for sure are coming; Talim and one more new character. The rest of the cast is basically going to be from Soul Calibur 1, given the timeline and the fact that Soul Calibur 6 is a retelling of Soul Calibur 1. It stands to reason that they would bring in the roster of the first game back. Whether Seong Mina, Hwang, Rock, and Lizardman come back as clones or original characters remains to be seen. It's unlikely, given that SC4 gave Mina and Rock a mostly updated movelist and Lizardman could have his SC5 incarnation, but it's not confirmed.
Another point: SC1 had 20 characters. 3 of which are rather superflous (Arthur, Edgemaster, Inferno). Soul Calibur 6 has 23 on the base roster. 2 fan favorites (Talim and Zasalamel), 2 newcomers (Groh, plus one other), and one guest. That eats up 5 slots, leaving just 18 returning characters. That basically fits the entire Soul Calibur 1 roster.
So with the DLC, there's four more characters. Well... out of the newcomer cast from SC2 through SC4, there's Cassandra, Raphael, Yunsung, Tira, Setsuka, Hilde and Algol. If there's only four slots, three would be left behind. Of course, if there's another guest character making an appearance as an exclusive DLC character with the purchase of a certain edition of the game, they'd probably fit into one of the main roster slots above. I suspect then, we would see Edgemaster getting the axe in favor of this new DLC guest character.