The problem with that is it's hard to introduce new characters when the main cast is already full of 8 people. Everytime a new character was introduced, it was with a smaller team. Even when Ryne was introduced, the first thing the story did was find a way to push the twins away for one zone, while Y'shtola was not in the equation yet, so the new character would not be drown under existing characters. Same for Zero. It's hard to imagine them introduce more than one (maybe two) main characters, and they will be in competition with the old ones.
I'm also part of the people that thought the 6.0 last scene was a declaration of intention for the characters, but it wasn't. I was slightly disappointed when I saw that half of the trailer was basically there just to say "Yeah, they all will be there". My biggest issue is that except for Estinien, all the scions have the same background : scholar from Sharlayan. It made sense with ShB and EW, but I wish we'd go back to more diverse cast, like in ARR (with Cid, Biggs and Wedge being important) HW (with Ysayle and Estinien) or SB (with Hien, Gosetsu, Yugiri, or even Lyse). I like (most of) the scion, but a little variety after 2 extensions would be nice. Keep some, add new characters, and then in the next expansion, take other, and leave the one you had in the previous one.
Now, I still have interest in the next extension, but I feel they just missed the perfect opportunity to introduce new cast member. I have difficulties to feel the adventure and going to the discovery of a not well known continent with 5 scholars of the world most advanced and knowledgeable country in my backpack.
