A PHBII (or something like it) after 2-3 years sounds OK to me, given their stated design goals (needs to fill a need, needs to be clearly modular). Optional like feats and multiclassing sounds OK, and it's the perfect place for stuff like entirely new classes (like the mythical "support-oriented non-magical character") or even new takes on old classes (new ranger!), races that don't fit comfortably into FR storylines, DM options that might've needed more page count than the initial DMG could dedicate, etc.
It's interesting because this sounds like it's 5e's version of the ".5ing" that has happened since roughly 2e. 2.5 = Skills & Powers; 3.5 = ...well, 3.5; 4.5 = essentials; 5.5 = "Here's a book of more options!" (you could even make a case for 1.5 = Unearthed Arcana!). It seems like a good use of that economic inevitability.
On the survey, I was mostly neutral about new options (put in a reminder that fast, easy, evocative mechanics are always better than complex ones, and to concentrate on the STORY behind a class rather than the mechanical niche. I've played dragonborn, gnomes, and minotaurs (huh...no "common race"!), and bards and clerics and sorcerers.
I harped on the lack of narrative cohesion for the Mystic some more. "I am touched by the far realm and so I can solve mysteries and punch hard" needs more explanation, more context, more presence, if it is to be the narrative.