The Diablo universe has an allready established set of orders, mercenary companies, tribes, clans and factions where all the classes come from. There can always be something new yes, but it should fit into the big picture. And why there are no non-human classes? Well if you have to ask, you haven’t been paying attention.
And nothing of what we know indicates, that there will be no Witch Doctors and classes along the lines of Paladins or Amazons.
We have the classes we know for starters, and there will he more to come in the future.
Druid, at the moment, as long as it is not clunky like the witch doctor in D3, and also necromancer, but I will probably try druid first as it has been so long (I did not enjoy D2:R feeling its game play too slow and… boring… after all these years away from it.)
One for single target/small cleave. One for crowds. One for generating resource. Two for monster resistances. And so on. Some of theme can overlap, but if you make your single target skill dmg type A, and your AOE dmg type B, it comes with consequences when an elite or boss is resistant toward A.
Then you can of course also have skills that empower each other.
Imo, a standard build should have at least 2-3 attack skills (2 skills would mean massive disadvantages against some enemies, 3 skills would still have significant disadvantages, 4 skills would start to be able to cover most stuff), that all deal a significant part of the total dmg (so no “The other attack only exist to buff your main dmg skill”.
Even more than that, they assigned different things to primary abilities for different classes, for example:
Sorc: Int = Damage, Dex = HP per hit (i.e. Sustain), WP = Mana gen, Str = ? (don’t remember but hit-effect empowerment I’d imagine)
Barb: Str = Damage, Dex = Fury gen., WP = Sustain, Int = Damage (Spectral ?)
I might be wrong (or forgot some of these) but think I also remember the Devs saying that Rogue had Crit assigned to Int
So yes, as you can see there is already a relatively healthy (at least offensively) dispersion of aspects of gameplay not only to skills but also to stats of character… I get if people get upset with the “omg Crit damage again ?” (Rogue on Int), but we don’t know (yet) what gets sacrificed for it
I’d kinda also imagine Necro to have one stat related to damage/endurance of Summons specifically (for example corpse % creation ?)… And yeah, the Druid is still the mystery above all (in more than one way )
Actually I’ll try to guess both of these:
Druid:
Int = Spirit re/gen,
WP = Damage,
Str = Sustain,
Dex = Talent buff and hit-effect duration,
Bear: Str = Damage, WP Sustain
Wolf: Dex = Damage, WP becomes Talent buff and HE duration
Necro:
Str = Essence gen and Physical strength,
Int = Sustain,
WP = Corpse creation and Summon power,
Dex = Summon sustain
(in more than one way 
)