So here is my take on the roster:
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I've added Talim to the far left side of the screen close to the good guys and Voldo to the right side. Inferno is in the center as an unlockable mimic around which the story revolves (we know he's in the game thanks to the Story Trailer). CaS is opposed to the random button as it usually is.
Astaroth, Cervantes and Lizardman have all appeared in story mode art, so they are all safe bets, Astaroth has even been leaked by Vergeben at one point. Another character leaked by Vergeben is Tira. All of those are villains, so they go to the right side of the screen. Astaroth is opposed to Maxi horizontally, Voldo is opposed to Yoshimitsu horizontally, Cervantes is opposed to Ivy vertically. Not sure about Lizardman and Tira, you can switch those if you want.
If I'm not mistaken, it has been officially confirmed Groh won't the only new character in the game, so I put the second new character (Groh's dad?) in the center. There is no free space left on the right side with villains and anti-heroes, but I don't want to put Groh's dad too far away from his son, so the center is the bast place to put him.
Which leaves us with only two slots on the left good guys side left empty. Most likely candidates are Seong Mi-na, Hwang, Yun-seong, Cassandra, Setsuka and Raphael.
Now Raphael is more of an anti-hero/villain type, while Setsuka was not very active during SC1-2 time, yet both are very unique, iconic and popular, so I delegate both to DLC along with a promised second guest. And that's all for season one DLC.
That leaves us with Seong Mi-na, Hwang, Yun-seong and Cassandra claiming two empty slots. I think getting both Hwang and Yun-seong is a waste of space, so only one can appear, which in my opinion should be Hwang, that leaves us with Seong Mi-na, Hwang and Cassandra battling out for two remaining slots. Well, Seong Mi-na and Hwang go together in the story also both are SC1 veterans unlike Cassandra, so they win.
I'm afraid Cassandra's only chance to get in SC6 is in the potential second season pass or as a CaS clone of her sister...