IMO, if they want to support more than the 8 classes that are in the PHB1, they would be very well advised to rethink the number of classes in the PHB1. The problem is one that was faced by supplements for 3e - no print book can assume that the player base has access to any other book (or, in this case, the DI). So, if they want to use a class that's not in PHB1, they essentially have to reprint enough of the detail of the class to enable people to use it. That eats up page count on recycled material, which is a major issue in print. This became a huge issue for Dungeon, in particular, which found itself reusing the same handful of high-level monsters over and over again.
Plus, of course, anything that's not in PHB1 may well not be in the OGL, which of course means no third-party support either.
All that said, my predictions for the 'magic 8':
Fighter
Cleric
Wizard
Rogue
Warlord
Sorcerer
Paladin
Duskblade (or Gish, or whatever they want to call it)