There will be 2 new class coming in the expansion and this is guaranteed. Do you see every people playing the same class ? No, people must have a choice and Blizzard has always been about "diversity".
Where has it been said the expansion will have 2 classes? Could be more. I have heard they want to do multiple expansions, so they could very well just add one class per.
At the very least we need a STR class, we already have 2 DEX and 2 INT while STR only has 1, so it's a natural choice.
Given the Barbarian is based around the style of attack-attack-attack and wades into battle wielding huge !@# weapons (one in each hand even), I'd like to see a second STR class that is more defensive oriented. Something along the lines of the Paladin from D2, designed to go into battle with a shield. Class specific weapon could be maces, hammers and possibly swords, the off-hand would be shields and the armor could maybe be pauldrons or something.
I'd go for a vit-based class who's resource management is based on his own life supply XD
OOH, I like it. Blood mage: has a resource (blood, vitality, whavever) than is used to cast the more mundane abilities, but the real nuke abilities are cast from hitpoints. To counter-balance this, the resource generation skills would include several life steal options
I will definitely add my name to the list of those who want a necromancer. The Witchdoctor does have some stuff that is like the necromancer (a few summons, poison, some curses), but there's plenty of differences, Summons are animals and zombies rather than skeletons and zombies (so as much difference as there was between necro and druid summons in D2) and there doesn't seem to be as much of a focus on summons (or at least not permanent ones), and there are no bone attacks; the curses area seems more based on creating terror where most necromancer curses are more about debilitating an enemy. The witch doctor is a witch doctor, necromancers are mages.
I will grant pet classes often pose a problem. Besides the lag issue which doesn't seem to be as much of a problem now from what I have seen, a bunch of summons does cause trouble in tight confines. beyond that, being tied to corpses for summoning is a bad idea, it makes handling bosses and champions something of a difficult affair when the supply of bodies is limited, and D3 boses are instanced, so you can't leave momentarily to restock at your last killing spree, and of the bosses that do summon minions, it's usually only one or two minions at a time.
Still I think if they could reduce the number of summons to three to five max (down from 17 IIRC) and remove the corpse requirement and we have something to work on.
I think part of the differentiation could come in the form of giving the necromancer a bit more in your face abilities (witch doctor fights more from a distance, and all of his poison stuff is lobbed from a distance). If they bring back poison dagger, thats a melee ability right there, and there's bone armor for protection, plus poison nova is a bit more in your face, add on a few other things that work best in the thick of things similar to how the wizard has stuff like wave of force, frost nova, and spectral blade (for example, a rune for teeth that sends teeth in a 350 degree circle rather than forward; add the ability to conjure a bone spear [an actual, handheld, melee spear, not the bone javelin called a spear]). A caster that is less squishy would be great, I know there's a sort of melee wizard build, that I sometimes hear about, but both INT based classes are very much glass cannons.
Skills would be reduced , obviously. Golem would definitely be one skill with runes, and I imagine skeletons would be too (skeletons is base ability then runes could let you raise skeletal mages or archers[didn't have those before], additional runes could let you raise fewer, more powerful skeletons or alternatively zerg your way to victory, and they could also replace skeletons with rotting corpses, do poison damage and are meatier, but hobble a might slowly). We can have poison nova, and poison dagger as well as teeth, bone spear and bone spirit, curses would likely be reduced a lot, and due to the corpses during bosses problem, I imagine, it would actually be a good idea to leave corpse explosion out. if bone wall is retains, bone prison would be a rune.
The only major overlap I can see (the pets is a minor overlap) is actually in equipment, specifically class specific gear. Bringing back shrunken heads as the off-hand would overlap with mojos (though if they make necromacers a more hardy class that wades into battle, bone shields would fit), necromancers are shown to have an affinity for daggers, which is already a class specific item from witch doctors, and I would imagine a good class specific armor would be the skills of demons and animals, which is awfully similar to WD masks