Mechanics have been changed a good deal, yes, there's not really much refuting that, but the story will be almost exactly the same, with more details filled in. Details that were only in bios before, or only available via external sources like art books, will have their place in the game's actual story mode. And not just like, cutaways in arcade mode, or the one true story and everyone else doesn't matter approach, but everyone's individual stories.
With that in mind, having as many returning characters as possible is the best way to take advantage of this, since the more new characters we have, the less characters we get to flesh out properly. You can argue that we'll see the characters in cutscenes, but not actually have them playable, but that just lessens their value, makes it seem like they don't matter.
Every character that has existed in this series is important on some level, and they all play their parts. Some less than others, to be sure, but those lesser impactful characters, it shouldn't be seen as an opportunity to leave them out, but to actually bring them in, to put everyone on an equal playing field, to have details that were only speculative before be given full undisputed canon status instead, that's the intent with the story mode that is going to be in SoulCalibur VI.
Grøh's inclusion (and his leaked father) is intriguing, at that, don't get me wrong, But that he's so out of place in itself is perhaps his plot point, as has been speculated that perhaps he is a time traveler, given his far more modern look that absolutely does not look like it came out of the sixteenth century, it's a well-reasoned enough possibility that it could be true. Time travel is an established concept, as seen in SoulCalibur V, where Edge Master sent Patroklos back in time to undo the killing of Pyrrha.
Grøh may very well have taken the Astral Chaos under his control to go back in time to prevent the future where Soul Edge influences the world, and that could be the Aval Organization's goal, to change the past. They will probably ultimately fail, as most time travel plots go, but he may just be the justification that is taking us back to the past to repeat the events of the previous games in the first place. We won't know his story until the game comes out, to be sure, but if this is the way of things, then him (and his father) being the only new character(s) makes sense.