The problem with new classes is making new animations, new armors, new class quests, the intros and text crawls, maybe even a new intro planet for an extra new class (origin planets contain two classes), and above all else, more voice acting. Sound files add a whole lot to the game. It wouldn't be practical to add two things so huge. An imperial trooper would basically be a Republic Trooper only Imperial aligned, so there'd be no real distinction from the 'new' trooper from a Bounty Hunter, Dark Jedi from a Sith, etc.
true but doesn't hurt to just out ideas out there for the hell of it