As far as I know, 3 classes have officially been announced thus far.
And they are...
Witch Doctor (Necromancer replacement)
Barbarian
Wizard

If Diablo 3 follows the initial pattern of the 2nd game, that leaves 2 more classes available for launch. With 2 casters and one heavy melee class, I think it's safe to say that an amazon (ranged/melee non-caster) class is a safe bet, perhaps under a different name. That leaves room for one more, and probably is the more difficult of the 2 guesses. The other remaining classes (including the Lord of Destrucion Expansion) consisted of the Paladin, Druid, and Assasian. (All are essentially hybrids offering melee or caster/ options) Of these three, I'd prefer the paladin to return, but would probably be more interested if they ventured somewhere with a new class altogether...
Being able to play as an Archangel or something? (Out there I know, but shaking things up a bit would be nice, I think they made the right step with the druid class in LoD)

I don't know about all of you, but I would have preferred the older necromancer cosmetic over the new witch doctor...  I'll have to wait and see though! :)

This list does not pertain only to games made by these companies recently, but throughout their history.
It's also strictly based off of my opinion and personal biases (and is limited to and based off of games I have played), however, I'd love to hear and discuss why you agree or disagree or any comments you wish to leave. :)

 1. Nintendo- Zelda, Mario, Metroid Series (Ik Metroid has alot of outside involvement, and now with Team Ninja, but I had to include it)
 2. Blizzard- Starcraft/Diablo Series (I won't mention WoW, oh woops!)
 3. Rare- Goldeneye, PD, N64 Banjo titles, Starfox 64 (essentially all N64 Rare titles)
 4. Capcom-  All things Resident Evil :)
 5. Bethesda- Elder Scrolls, and Fallout 3 I suppose...
 6. Ubisoft- Splinter Cell and PoP Series
 7. Epic- Unreal and Gears of War Series
 8. Bungie- Halo Series
 9. Konami- METAL GEAR!
10. Bioware- Kotor/Mass Effect Series

*EDIT 7/6/09
*Thanks for sharing everyone! I know 10 may appear excessive, but I suppose it depends on how choosy you are (hehehe)
Updated my list by posting some highlight games for each developer, as many of you did...
And Firaxis certainly makes an honorable mention up there, I can't neglect the awesomeness of Civ!