As long as the class distribution holds the way it is now, Blizzard could add more classes to the game. The main obstacle on that side of things is that there isn't much left in Warcraft to be made into a class. Everything is pretty much covered.
Which by the way is sort of where we are with races. Everything is pretty much covered on that side of things as well. Like someone said above, when you start considering Murlocs or racial retreads like High Elves, its probably time to stop.
I think we'll be getting a model revamp next expansion. There's already evidence in the game that Trolls, Orcs, and Dwarven models are pretty far along, because we have NPCs utilizing those models. I'd also love to see sub-races. Like Humans could become Vrykul, Draenei could become the Broken, or the Forsaken could become undead High elves like Sylvanis. That would be very cool, and add a lot to the existing races.sub races? i assume will get them eventually but thats a heavy undertaking, every race would require one otherwise it would be unfair, and all the races that would be added would require remodels to be as good as the normal race models and not to mention some races would require new lore and even new beings to be created to even have a sub race, this undertaking would be similar to the model revamp were nearly confirmed to be getting next expansion.
a model revamp could be a good substitute but thats a one off thing, atleast for a long time.
were getting new races whether you like it or not