will there be a class for a sith trooper?
In functional terms how would a Sith Trooper be different from a Bounty Hunter/Republic Trooper? The Bounty Hunter is already the Imperial counterpart to the Trooper, and BW would not create a whole storyline just to duplicate an existing class' role.
That means the Sith Trooper would have to be something more than Bounty Hunter with a new storyline and look to justify the work. And then the Republic side would have to get a version of whatever the new Sith Trooper would bring to the game for their side, complete with storyline, to maintain parity. So, barring the addition of a neutral class, which a Sith Trooper obviously would not be, adding a new class would automatically mean adding two classes to the game.