DRK as a tank would be acceptable. I imagine them as the Death Knights from WoW... except cooler.
BLU I believe Yoshi-P said that he didn't know how this would work in the game (given that monster skills would be ridiculous to try and work into a class).
RMG Is the class that Yoshi-P wants to implement the most apparently but he said he wouldn't now where to begin to balance it
SAM would be a good class to throw in and would be pretty fun to play I think. Tank or DPS it would be pretty cool regardless.
NIN probably going to end up in the game eventually and will most likely be a DPS, ranged DPS or Melee, we'll see (think Yuffie).

All the other classes still have a lot of speculation around them, the ones above just seem to be the ones that get brought up the most, lol.