I'd be happy with Final Fantasy VI level interaction... IE in large gatherings the 8 interact with one another, make decisions together, spar or develop relationships. I think it would be a missed opportunity to essentially have them act as shells around one another for the whole game and instead have interactions with the NPCs in their respective chapters. I mean, there are many, many games which have the narrative flexibility to handle 8+ playable characters, NPCs and antagonists... (the Trails series comes to mind).
What I am curious about is, will there come a point in the game when there is a forced single narrative? Think of Final Fantasy VI when the characters had to go on separate adventures (Terra and Edgar go in one direction, Sabin goes off in another and Locke on another- this was fantastic for character development, relationship building and narrative exposition... and it's rarely been done that well since!).
If this is a true spiritual successor to Final Fantasy VI, I hope it's not just in graphics and mechanics only, I hope there is some tribute to the inspired narrative structure of VI.
Then again, this is modern day Square Enix, where story is either vacant, convoluted or a tagged on afterthought. At some point the company will need to wake up and see how much people crave a well made story in JRPGs- the tremendous affection for Trails games and WRPGs like Witcher and God of War.
FFVI works because there's a singular story driving things along and characters go in and out. Characters are there to serve the story, the story doesn't serve the characters when they're not involved with the bigger conflict.
And again, the spiritual successor quote is a misquote and the producer was talking about mechanics only.