You realise this is not a complete carbon copy of the SC1 storyline? If it was, it would be hella short lol. The existence of Groh as a new character should have made this clear to you already but I guess not. This is simply a retelling of the major plot points of SC1 - they can easily rework it in order to fit characters like Talim, Raphael, Zasalamel etc. into the story.
You can say that, but it still seems like, yes, for the most part, it is a retelling of SoulCalibur I storyline. There are going to be far more details and events shown, but it's still the original SoulCalibur I storyline. Grøh's parts will be brand new, for sure, but from what we've seen so far, everyone's story who was already there is basically already the same. Tira's story can exist because she has a known backstory that hasn't actually been seen, so if Tira is in SoulCalibur VI, it will be telling her story as a Bird of Passage, not as a servant of Nightmare/Soul Edge, perhaps ending with setting her on that path, but there's plenty of room for her other story to be the main focus.
It doesn't seem like to me that they're changing any real details about anything that already happened and is known, they're just fleshing everything out a bit more. Even with regards to Grøh, it was never said explicitly that he did have a rivalry with Kilik, but it wasn't said that he didn't, either. It perhaps has some implications on Kilik's darker side, things he'd rather forget, things that he only tells his closest friends, Maxi and Xianghua.
So, yeah, Talim, Raphael, Yun-seong, and Cassandra could be included in the game, but not necessarily to play roles in the story, but they could only if they have places to be that can be justified by known or extrapolated events, but of those four, the only one I personally see having a story bit would be Talim, since she's number one on the character poll, so they'll give her a little extra love. She'll be on that short path at the end of the timeline, the line next to Zasalamel. The other three can make it into the game, but probably would be among the three characters that don't have stories. Setsuka definitely doesn't have story, though she'd still make it in as a DLC character, who wouldn't factor into the main story mode of the game anyway. Same goes for Hilde.