Honestly with all of the omitted content in the original game, I would enjoy (and almost expect) hero classes for each class. But with Blizz, I have learned to expect the worst and only hope for the best.
I think a given is that were are going to see at least another Strength class (there are 2 Dexterity and 2 Intelligence so far).
Also Blizzard has set a pattern in their characters:
(Just at looking back at Diablo 2 and LoD)
Diablo 2: Paladin, Barbarian, Sorceress, Amazon, Necromancer, Assassin, Druid
Diablo 3: Monk, Barbarian, Wizard, Demon Hunter, Witch Doctor, _____, Some sort of shape shifter.
Granted the Monk and Paladin aren't completely the same. They are both "holy"-like warriors, but everything else matches up quite well.
My opinion, we will see a Strength class that follows a shape shifting play style (my best educated guess). If they release two characters with one expansion, then the second character could be something like a stealth fighter play style maybe (no clue).
I would like to see a sort of greek hoplite sort of class. Perhaps it could have insane shields/physical resistance passive and it refills its attack resource by succesfully blocking/absorbing damage instead of landing attacks. Hell im pretty sure there's a spear called the 300th spear or something like that, maybe its a sign.
I think a Warrior-mage class would be cool with +.5% dmg from str and int, but i think we are looking at a less magical or nature class for next expansion. btw they have said they want to have 2 expansions (prob one class each). Necromancer would be cool but it's not gonna happen.
There was a blue post a while back saying they made the WD sufficiently different from the Necromancer so they could hopefully bring him back in an expansion. Not a confirmation, but having a long-term plan like that is a good sign.
If it's going to be two years between expansions, I fully expect two new classes per expansion. New classes really expand the replayability of the game.
Someone mentioned corrupted druids, so I thought I'd expand on that. It would be interesting after the events of D2, their reliance on animal instincts overpowered them and they lost the ability to revert to human form. A third caster form could be added (NOT moonkin).