From the AMA:
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.
