So let's speculate on what the last 3 classes will be. There are 2 Sith and 1 Republic remaining.
One I am completely sold on for the Sith is "Imperial Agent" or something like that. Along with the recent comic issues putting this potential class on display, we have this leaked forum icon
That pretty much all but confirms some kind of Sith Agent/Spy/Assassin class.
I do not think there will be any more force classes announced though I've seen quite a few people who still believe there will be.
For the other Sith class, I think there are a couple possibilities. I would still love to see some kind of Mandalorian class (yes I know they are a faction) but that seems to be getting more and more unlikely. The other possibility I see is some kind of Sith commando, heavily armored class.
For the last Republic class, I am either thinking some kind of Tech Specialist, or Spy. Really though, I am not satisfied with either of these ideas for the Republic, as they are just not anywhere as iconic as the other classes are, which is what BioWare is really going for. What do the rest of you think?