I agree that anything like this wont be out for a long while since Blizzard wouldn't want to mess with their cash cow WoW, but think of the gameplay possibilities.
You could play as a ghost and hook up with a bunch of other ghosts for a demolition mission deep behind enemy lines. It's up to you to make sure that some key installation of the enemies is taken out so an important assault doesn't meet overwhelming opposition. Mabey as a zealot you and a regiment of other zealots have to hold an outpost against a zerg infestation untill reinforcements can arrive.

Sorry to be so unaturally adamant about this but I guess I'm just buzzed about the idea, even if it might not happen for a very long time.