Will any other subclasses from the supplemental sourcebooks ever make their way into the Game?
Certain classes have more choices and variety than others if we limit ourselves solely to the Player's Handbook.
Any chance we might see more in the future post-game launch?

From the very deffinition of that word, everything is "possible".

But nobody knows.
We were promissed all classes and subclasses from PHB ...
And we were told that they are looking at other source materials for inspiration ...

But that is all we know for now.

On the other hand, moders seems to be quite active allready, since we have some subclasses for Warlock allready. (mmm Hexblade. :3 )
So, if mods are good enough for you, i think its safe to presume that most classes even from other sources will be possible to play.

I am hoping the oath breaker will be in. Especially considering the oath breakers that left Helm…