Apparently a lot of information went out....at least in the form of development talk....about the forthcoming Pathfinder Unchained. ENWorld was kind enough to gather the details all here, but the gist of it is that this book will focus on revisiting old sacred cows of the 3rd edition mechanics with a fresh eye on design. Things which caught my eye included revisions to make the barbarian a better play experience, lots of skill revisions into adventure/non-adventure tiers, "messing with the action economy," and rogues getting danger sense instead of trap sense.
They can spit it all they like as a "not Pathfinder 2nd edition" tome, but it sounds an awful lot to me like a way to print a bunch of revised rules as an optional book and see what sticks an what doesn't for a 2016 Pathfinder 2E release.
After reading through the discussion, I have to admit this book may (if it delivers what is promised) drag me back to try Pathfinder again. Maybe.