I'd rather see WotC embrace the Lego-block feel of 3e style multiclassing and release lots of classes, with variable length, with stacking class features. Let them function almost like talent trees. Down with symmetry, embrace the madness!
I think 3e multi-classing is brilliant. What it does is allow you to define some salient points in the design space in a very flavorful and non-generic sort of way, and then all those other fuzzier concepts in between that your base classes can't quite cover should be reachable by blending two or more base classes. So yeah, I love the way you can tinker with multiple concepts to create something new or just something that fits the idea more precisely.
If the route of 6-8 generic classes with expandable subclasses/archetypes isn't followed,
I don't think we need to be as tight and generic as 6-8 classes. I think that a good class based system can be made with anywhere from say 3 to 15 base classes. Any more than that and there is more mess than functionality. Each of the base classes in turn should be able to support 6-10 archetypes via customization in abilities, feats, skill selection, and class features (which are essentially bonus feats with a limited flavor specific selection).
My current list is:
1) Champion - Warriors that acquire power by faithfully serving some cause (a Paladin, for example).
2) Cleric - Workers of miracles whose power comes from service to a deity.
3) Explorer - Expert travelers and generic adventurers.
4) Fanatic - Warriors that acquire power from inner strength, passion, and emotion
5) Fighter - Warriors that study martial arts and wars, and hone their skill with weapons to the highest degree.
6) Hunter - Warriors that study particular prey rather than particular weapons, so as to excel in defeating a particular foe.
7) Paragon - Exemplars of what is best in a particular race, they seek perfect themselves and achieve some great destiny. (Design isn't yet complete.)
8) Rogue - Skillful masters of all kinds of deception and skullduggery.
9) Savant - Masterminds who excel in knowledge, perception, crafts and the arts. (Design isn't yet complete.)
10) Shaman - Spell-casters whose power comes from commanding, bargaining with, and serving spirits and other magical beings.
11) Sorcerer - Spell-casters having innate magical power as the result of some accident of birth.
12) Wizard - Spell-casters whose power comes from gaining esoteric knowledge about the workings of the universe and honing their minds to command secret and hidden forces.
I consider those 12 classes likely to exist in any fantasy setting. The only exception would likely be the 4 core spell-casters who may or may not exist depending on the assumptions the setting makes about magic. However, between the 4 I think they cover most approaches to magic you are likely to have, as for example the Sorcerer is sufficiently generic that just about any spell-caster can be emulated from one (psions, channelers, jedi, etc.). My idea PH would have those 12 classes as core, and 3.X style multi-classing with a few tweaks to support multi-classing with a spell-casting class (that doesn't work in core).
I also have 3 non-generic base classes I'm supporting for reasons specific to the campaign world.
Bard - The D&D Bard class, slightly beefed up, supported mainly for back compatibility and iconic status.
Akashic - The Monte Cook arcana evolved class, slightly beefed up, supported because it is essentially a racial class in my game to develop the flavor of my homebrew Idreth race.
Feyborn - Homebrew class where you gain spells as spell-like abilities. Supported mainly because it is essentially a racial class in my game, because Pixie, Sidhe, and Changling are core PC races. Essentially, lets you become a powerful fairy lord.
All three classes are Jack Of All Trades, which isn't surprising because the pure specialists are all core classes. All conceivably could be played by multi-classing and a few supporting feats, but I didn't go that way because the implementation is a lot cleaner this way and the classes are central enough to the setting to be iconic. Classes like these would appear in Setting books and be explicitly marked as being exclusive to a setting, and not necessarily available outside it (consult your DM). However, any thing in the setting that can be made for core classes probably should be. Even setting specific base classes should be pretty darn rare.
I also have three NPC classes I use when I don't want the NPC to be particularly special. These are purely matters of convenience. I could support all of them with the above concepts but its just easier to do ordinary types with weaker less balanced classes.
Brute - Tough NPCs but without a lot of weapon skills.
Expert - Knowledgeable NPCs (a scaled up version of this class is going to be the Savant, as soon as I can figure out how to keep the combat relevant above 5th or 6th level)
Commoner - Very ordinary non-combat types, who have had little experience or opportunity for training in anything
).
