For the next quantum leap I think we will see something like D20 Modern: a half-dozen or fewer base classes each concentrating on a very broad archetype, with plug-in talents (which will be expanded to be as versatile as the feat system) and bonus feat slots for high customization. They will all look like the 'vanilla classes': Fighter, Wizard, Cleric, Sorcerer, or NPC Expert (which will supplant Rogue). All the specialized classes with lots of class features (Barbarian, Monk, Bard, Rogue, Paladin, etc.) will be transformed into Prestige Classes or be stripped down into component talent chains.
The current paradigm of "levelling up" that raises everything across the board (BAB, saves, SP, etc.) will remain, since to do otherwise would necessitate some wonky stuff that may require everything to be turned into a feat (fixed-strength abilities you activate), skill (activated abilities that increase in strength), or talents (fixed-strength abilities that are always active).
I don't think we have anything to fear about a truly classless version of D&D. Eliminate classes altogether, and it will be D&DINO (D&D In Name Only). Call of Cthulhu tried to be a good classless d20 system and IMO is a total failure with its "choose your BAB, saves, etc." system, mainly because the game design of d20 is integral with a class-based setup (3 BAB progressions and 2 save progressions). d20 Modern was a better attempt at a semi-classless system.
If not a d20 Modern-like system, then we might see a mighty proliferation of 20-level classes, as we have already seen. D&D's multiclassing tends to produce stunted characters, especially at high levels. Adding in new core classes provides a way to have that hybridization without sacrificing the highest-level abilities of a class.
ith that said, WOTC's record of new classes hasn't been perfect (some of their new core classes make more sense as prestige classes). But most of the core classes in the total set formed by the PHB, Complete X books, Heroes of Horror, Miniatures Handbook (marshal & healer), and Player's Handbook II would make good "flagship classes" for D&D 4E. 4E's PHB2 could contain a healthy number of prestige classes and a few additional core classes. (I never thought it was a good idea to put the prestiges in the DMG and to include so few of them, but I suppose book space permits only so much .... even so, Web Enhancements galore should do well)