Well since it's a reboot they're free to introduce whatever new elements they want. I for one am dying for a brand new female fighter that actually adds rather than takes away.
Even if it is a reboot, it still is a new game in and of itself, and to leave out the ideas of bringing in new characters seems counter-intuitive to the concept of it being a sequel, much less named Soul Calibur VI. If Groh and his father are the only new characters in the game (and I'm not saying they should be, but if they are), then they already have a spot in the canon by way of being directly tied to the Aval Organization. Z.W.E.I. and Viola in comparison are shoddy additions aside from their polarizing gameplay, only because we never really found out what their deal was.
In and of itself, new characters are a wonderful idea and with the reboot, it is in a wonderful point in the series that quickly re-establish itself bring in new characters and add far better twists to the story that what SC4 did. I caution, however, that we don't really want another situation like SC4 had with Algol, being rather universally reviled. So while new characters are nice, we still want quality.
That said, however, we're not likely to get a number of new characters like SC2/3 did because of CAS development times and because of the single-player story modes. It took a solid year and a half for SC2 to come to consoles and it added an additional four more characters (Necrid plus the guests) brought back three, and took two older styles and put them on faceless characters; not to mention adding more new moves to every one in the cast. And addressed several bugs, and added three more stages.
And the sole single-player mode in the game was a Mission Mode with no story-specific cutscenes other than the ones added for the new characters.
If we could get that kind of quality and development time again, it'd be wonderful, but it's not going to happen. Soul Calibur is exclusive to consoles now, and without that Arcade presence, funding and extra development time for console versions (and keep in mind Tekken 7 is rather anemic in content despite a year-plus in the arcades; but it does have a really solid character roster and guests). We're stuck to basically two brand new characters, and like SC4, one of them is a boss character. At the very least, we're getting fan favorites who missed a game and some old favorites who haven't been seen in perhaps a decade.
And despite all of that I've said, I'm perfectly happy with being completely wrong on the roster, but I don't think I will be.