My Two Cents: Angel Knight and Jester
Here are the two classes that in one form or another I believe we will see as the final two remaining slots.
The Angel Knight
What is an "Angel Knight", you might ask? My idea for it would be a former angel that followed Tyrael in defying Heaven's orders and directly interfering in the affairs of the mortals. In order to do so these Heavenly beings have cut off their wings, robbing them of much of their former power but allowing them to live in the mortal realm of Sanctuary (no floating around and waiting for humans to come and do their dirty work; the angel knights get down and get gritty). In playstyle they would resemble the Paladin of D2, with a mixture of melee combat and holy powers, but would have a more compelling backstory. I can see these guys having a bit of an arrogant twinge, too, and being colored by the fact that the humans of Sanctuary eye them with suspicion, seeing them as little better than other "fallen" angels.
My main reason for thinking this is because Blizzard has an ongoing track record of bringing in veiled Christian imagery into their games, and there is still a spot left for a support class, and I can't really come up with a better healer that would fit into the Diablo universe (the people of Sanctuary don't really seem to have many healers around, outside of the occasional witch or herbalist). Also, all the Tyrael artwork that Blizzard has put out smells like foreshadowing.
The Jester
I imagine this class would be similar to Batman's Joker, except fighting on our side. Going along with the theme could be special types of wind-up bombs, crazy killer puppets, and a whole bag of traps and tricks. So why the Jester? First, if my memory serves me right there is a reference to a Jester class way back in Diablo I (Jester's Hat, or something). Also, every Diablo game seems to have some sort of roguish, stealthy character in it, like the Assassin and the Rogue, and we haven't really seen one like that revealed yet in D3. The only thing that counts against the Joker class would be that if it has all these puppet and bomb attacks it could turn out a bit too much like the Witch Doctor. Even has a bit of the same wily streak to boot. I'd like to see a Jester class to be sure because I bet it would be pretty interesting to see where Blizzard takes it, but I'm less sure about this one than I am about the Angel Knight.