I'd fully expect Larian to handpick things from the PHB or other advanced material at their complete discretion and convenience on a case-by-case basis (i.e. PHB stuff but if they don't like how a class or function plays, they'll go fishing from the extras).
It would seem that's the case.
There have been hints of other 5e sources being implemented into the game. Are you going to include material from Mordenkainen's Tome of Foes, Xanathar's Guide to Everything, and/or other player materials outside of the core material of the 5e Player's Handbook? Additionally, at full release, will all content of the PHB be represented (notably, (sub)classes and (sub)races)?
Swen: HB/MM/DMG are the base we are starting from but we are taking material from other books too.
From the AMA, so pretty old source, but it's probably still valid.
