I don't know about additional playable races, but they have said there's probably going to be more classes either as stretch goals or barring that possibly adding them in a DLC / expansion at some point. Bards, Monks, Psions are among those that have been hinted at. They've also confirmed there's definitely going to be prestige classes (e.g., Arcane Archer, Eldritch Knight, etc.). They said they want to devote a good amount of time to each class they implement (so for instance if they add monks, they'll have high quality martial arts animations rather than just using a generic punch).
The original Pool of Radiance gold box game had only six races and four classes. It's better not to overpromise and just ship an enjoyable, polished game. Hence, I won't mind if this game only has the current class/race mix -- they can save an expansion for the sequel. But we'll see.