In my opinion, there are not enough classes in D&D. Unless D&D were to go a more generic route ala d20 Modern the archetypes are too narrow and limiting and there's not enough of them. I'd rather have no classes than classes that don't quite get me where I want. Either that, or more flexible classes that could allow for very different builds. Both Midnight and Arcana Unearthed do this, by allowing the character to pick from a big list of class abilities.
That said, I think there's some real wisdom in here so far, though. First of all, what archetypes you use is entirely dependent on what default setting assumptions you want to emphasize. Reading through AU you can really see that -- Monte Cook didn't just pick up random archetypes, he picked archetypes that were appropriate for his setting, and crafted them in a way that gave them setting specific details.
Also, for what it's worth, I don't consider the cleric to be a fantasy archetype in the least. It's a D&D-ism pure and simple. At it's most basic levels, fighter, rogue and wizard are the only true basal archetypes. But again, unless those archetypes are represented by classes that are extremely flexible, or served by multiple classes that explore different facets of the archetype, I would hardly recommend shrinking to three classes.
I have to agree--the core classes present doesn't really cover all the bases. However, I'd prefer a shrinking & generalization of the core classes rather than an expansion & specialization of them--Palladium Fantasy, and to a more horrific extent (IMHO) Rifts, have oodles & oodles of Occupational Character Classes (OCCs), which (IMHO) are horrible when it comes to balance--despite the roleplaying impusle to play a Vagabond instead of a Glitter Boy, Apok, or Cosmo-Knight, the poor old Vagabond will die off soon due to the heavies that the GM sends to deal with his Mega-Damage dealing & taking party members.
Ideally, I'd like to go with the generic "core three" that I mentioned in my previous post--the Feat-intensive, Skill-intensive, & Magic/Power-intensive. Character creation will be slower, though--introduce a lot of choice elements & it inevitably takes up time. However, it'd sure be customizeable, esp. for use in various settings. Want a low- to rare-magic setting, ala Lankhmar? Easily done! Want a high-magic, everybody-except-one-or two-classes-has-some-sort-of-spellcasting-or innate-magical-ability style campaign? Voila!
I see the cleric with a description for the class provided in the 1st ed. AD&D PH (IIRC)--based off of the religious orders of knighthood from Medieval times, sort of like the Templars or Hospitalers. Whereas the cleric is a mix of fighter & priest, the paladin is a specialized variety that's even more fighter & less priest.
With that said, I think that a generic magic/power-intensive class could cover wizards & priests (and psions, to boot). Besides, what if you want a campaign with no arcane magic at all (and thus, no wizards)? Or, ala the Ultima games from 4 on up, you want arcane magic only (thus, no priests & divine magic)? Easily handled by the magic-intensive class--they have 1 general spell list that covers pretty much everything.
However, to expand upon the 3 core class idea, I do think that ready-made templates should be available--if you want to quickly make upa particular theme/class idea, then here it is, with all the options pre-chosen for you. Want to be a Fighter? Then here's a pre-made option using the Feat-intensive class. Want to be a Sorcerer (in the 3.5 D&D sense)? There's a template for the Magic-intensive class, with a limited spell list, increased spells/day, decreased spellcasting requirements, and limited spells known selections.
But, I think that the execution of this will look like 3.5E D&D meets d20 Modern meets d20 CoC meets Skills & Powers (from 2nd ed. AD&D optional rules). In other words, a math-messy, convoluted nightmare. There will be some players who take up the ready-made templates for ease of use, but there will always be those few players who want to build their own PC, trying to figure out the best way to abuse &/or beat the system in order to create the "uber-PC."
(Sigh.) I know I don't want a plethora of core classes to handle every single permutation (e.g., "the rogue & the pirate classes are pretty much the same, except that the pirate can also use a scimitar, has Swim as a class skill, and doesn't get Uncanny Dodge until 2 levels higher than the rogue."), but I'm wary of what a free-form, build-it-yourself system might bring (e.g., "If I take all 12 of these class & spellcasting limitations, then I can cast all of my Evocation spells with a +15 to to their DCs!") Essentially, I don't want either a Palladium or GURPS. Just a simpler, more flexible version of D&D.
Hmm . . . to tell you the truth, I think that what may work is a reduced set of core classes (either the basic 4 or the 1-per-stat variety mentioned below), but a good number of prestige classes. However, these prestige classes should have relatively low-loevel entry requirments (i.e., make it possible for a 2nd or 3rd level character to adopt the PrC).
Thus, perhaps the paladin PrC would only require a PC to have 1 level of fighter & 1 level of priest (effectively meeting the requirements at 2nd level, & gaining the PrC at 3rd level). Or, the druid PrC may have really low requirements so that a 1st level priest, with the right skill, AL, domain, &/or feat selection, can meet the requirements at 1st level & gain access by 2nd level.
Sorry for the long post--it's one of those thinking-as-I'm-writing posts. Hopefully my point came across (if I even put it in there

).