Only 5 classes they've said over and over and over and over and over again.
Maybe more classes in an expansion.
Maybe.
For those following this closely: Wilson already stated that the 5th and final class will be a second returning class.
http://pc.ign.com/articles/101/1017305p3.html
IGN: You did mention that it's being revealed next year, and that it's someone from the previous games?
Jay Wilson: I did say that. I probably shouldn't have said that, because we're not sure that's going to be the case. It's just logical, that's what we tend to do. But it may or may not. I'm not gonna answer it, anyway? [laughs]
Best money is on the Rogue/Amazon.
But you never know. They may throw class mechanics to the wind and bow to public support and bring back the Necro. If they bring the druid back he'll be more shapeshifting and less summoning. Those 4 seem the most logical to me at least.
I would say the number 1 most likely is the Rogue.
2: Amazon
3: Druid
4: Necro