Sorry for the title, I just don't want this to degenerate into a silly brainstorming with ridiculous suggestion and joke class
Ok, so let's start, we can safely assume that there will be new class(es) introduced either in expansion pack (might be more than one) or if Blizzard want to experiment in a paid DLC, personally I like the latter as then we don't have to wait for everything in expansion pack.
So which class niche can be filled? Late January Blizz revealed their big 'system change' in which they overhaul the attribute system (again), and now we have 2 class each relying on dexterity and intelligence for their damage, leaving barbarian with a monopoly over Strength. So it's very likely that the new class will use strength as his primary attribute.
Generalising a bit, currently we have 2 clear-cut melee classes and 3 ranged classes, so it's likely that the next class will either be melee or hybrid (ala amazon or druid)
Moving on we also knew that another gaping hole we're missing is someone that's proficient in shield and I just don't see barbarian as a sword and board guy and he doesn't have shield slam :P
Also another element we're missing is holy damage, after all Diablo 1 and 2 all have that and we can call that another essential element in the Diablo universe.
From above it seems that Paladin will fit the bill really well, but we already have a templar and it seem that Blizzard prefer exploring new classes than returning with previous ones, so I think we can cross him out too...
So finally here is my guess, a strenght primary, melee hybrid, shield user, holy damage, and new class: a Fallen Angel class!
that will fit above criterias, satisy Blizzard desire for something fresh and will be really appealing as a paid DLC, there might even be a reason to hold fallen angel class until after release as we know something will happen concerning heaven and tyrael and this class by then could have the lore to back his story up.
I think the classes should be nephalem in origin or else how would they benefit from nephalem valour? But seriously, I don't want to play an angel, demon, fallen, or any combination of the above. We already have a class with glowing eyes and "demon" in its name.
I like the idea of revitalizing the druid or necromancer class, but since Blizzard wants new classes, I'm guessing they won't do that. But, I do think the new class will either involve summons or shapeshifting. The monk already has the holy theme covered (although I wouldn't mind another religious class, with a different religion; i guess that's what the druid and paladin both were). The barb has the shield theme kind of, but who really cares about a shield bash anyway? I wish the barb had one, but he bashes much better with an axe anyway.
Considering that the new classes will only join in act 5, at least lore wise (much like the assassin and druid)
then maybe we'll see something like a Peace Warder from the Triune should it return in D3.
Would be much like the paladin, it would be from a corrupted order but not aware of the corruption within it.
You know, this got me thinking...wouldn't it be cool if the player became temporarily corrupted and/or hallucinated and went hostile on allied NPCs before being cured or coming out of it naturally, only to realize you've just killed Deckard Cain?