I actually like this idea. But inside of being a whole new class I think it should be a race unlock. If the ghost were to become a class it might screw some people over of they became one without knowing the consequences. As a race people could keep their original class while going through the experience of a ghost. The stories will have to be tweaked a bit but all of that can he added into a full blown expansion.
I also thinking the requirements of unlocking it should be higher. The alignment sounds good but the requirements are simply too low. Most people could unlock this either by the time they leave their starter planet or as the land on the capital. Instead new levels of alignment should be added for this unlock. Such as light/dark 6 or even 7. That way most people won't unlock it until they are either at the end of their story or close to it since each level of alignment seems to double the points needed. So for a alignment 6 you would need something like 40,000 and 7 would be 80,000.
This could also lead into racial buffs instea of racial emotes
My only thought would be that it'd be easier for BIOWARE to build new stories on already existing ones, rather than go back and restructure. I agree that unlocking the Force Ghost should be a difficult task.