Correct me if I'm wrong, but we have already some content from non-PHB sources.
Some examples :
- Githyanki (MtoF)
- Flind, Spectator and other monsters (Volo's Guide to Monsters).
That's not a lot, but it shows, I think, that it is possible in the future to have more non-PHB content in the game.
But, first, Larian Studios must add what is already in the PHB (gnomes, dragonborn, barbarians, bards, monks, etc, etc, etc).