The reason why I parallel Smuggler with Bounty Hunter is that they are both the semi-legal classes. A Sith Legionnaire makes sense because there needs to be a parallel with the Republic Trooper.
The Agent/Spy classes are a definite possibility. It'd also be cool if they had the unique ability to hire random mercenaries or bounty hunters.
Subclasses...skill trees...when it comes down to it, I'm betting that there will be the whole Consular or Guardian choice system. This is how much people function with the Force user class system. I just hope that they don't revive the Sentinel class: completely pointless attempt to make a middle ground that was just plain weak.
Prestige classes are a must. You should eventually be able to become a Jedi Master, Sith Lord, Sith Centurion, Republic Commander, Elite Smuggler or Bounty Hunter...etc..
As of 3/14/10, TSL is restored. The Sith Lords Restored Content Modification by Stoney and Zbyl has been finished and can be downloaded here