Unfortunately, the class with the most potential for this new reworking of the Diablo engine, the Assassin, is so forgotten that she's probably tossed completely. After seeing how the Barbarian is being thrown around as operating in combat, characters with a variety of melee skills for wading through hordes are going to be able to work quite well...sadly though the Sin just isn't as popular.
Keep in mind also, these classes need to have a feasible story for why they'd be around. After 20 years of general peace, how many evil-smiting Paladins are going to be running around Tristram? Even if they're added, I forsee an unfortunate reworking of them into the cliched "lol u died i'll rez j00" MMO role.
I'm really happy to hear about the Necs potentially sticking around, since they're my favorite by far, but the Witch Doctor is so similar I don't see how they'll co-exist too well without overlapping in their roles and playing styles. I mean, a Witch Doctor can at the present call up a wall of zombies...awesome skill, but if there was intent to be a Necromancer class, why wouldn't they put that skill on the Nec and not the Witch Doctor?
Amazons are so incredibly popular, I see them as almost having to make an appearance, although Jenny's right and they'll have to adapt them to a gender-neutral role...and what do you get when you take out the female aspect of Amazon idealogy? Barbarians who use pointy sticks and bows instead of hulking axes. I'm afraid they'll merge Amazon with Barbarian and just give them a larger range of weaponry choices.
And furthermore...with the Witch Doctor, it almost feels like a merging of Necromancer and Druid...we have a natureboy who uses pet spells, disease, curses, fire spells, and limited control of undead. Kinda infringes on the ideas of the Druid as much as it does the Nec.
If you ask me, I see the four integral classes that are likely to come out of this as being;
Barbarian - All-around battletank, large variety of weaponry for melee smashing.
Witch Doctor - Natureboy/Spirit World manipulator...caster class with pet spells.
Mage - Generic caster class, probably heavy on the elemental magics like Sorceress was.
Ranger - Solution to Amazon's gender specificity. In some games, Rangers use javs as well as bows.
That leaves a spot for another opening class...talks say Necs are staying, but I can't see that working in the same space as a Witch Doctor. Paladin would be a reasonable fit for final complement to a party, with group healing and vs undead skills, but who knows really. Blizzard's pretty creative, and they generally don't screw their loyal fans by keeping them from things they know and love...so we'll just see how things roll later in development.