Hmm...
Perhaps the game shouldn't have "multi-classing" as such, but instead should allow a character to take a certain number of cross-class powers? So, the Wizard could take some "item crafting" powers from the Artifice source, or pick up some "combat expertise" powers from the Martial source, without the need to explicitly multi-class. (Or, of course, he could take both.)
And, of course, if the character picks up enough cross-class powers from another class, perhaps he should eventually become eligible to retrain his underlying class, so that the "Wizard with lots of Fighter" eventually becomes a "Fighter with lots of Wizard"?
Just thinking out loud and riffing off of this same idea in a slightly different direction ...
Suppose that all the current classes are the multiclassing part of the game, and the core stuff goes into something else. For sake of argument, let's call it the "Adventurer" class, though it really is only a class in comparison to previous versions, and to distinguish characters who aren't really adventurers at all (e.g. experts, nobles, merchants, soliders, etc.)
Being an adventurer means that you get adventurer stuff as you level. This includes some of the base scaling stuff (hit points, base attack numbers, etc.) to the extent is present at all. But it also includes base picks across all skills, some general "feats" however conceived, and even some picks of commonly available "powers".
Then from adventurer you multiclass into at least one class, or more if you want. This multiclassing is alongside your adventurer class, much as the 1E elf fighter/mu, though your XP total is only on the base adventurer side. Your multiclass picks are along for the ride.
Say you pick fighter. Now you get some flat abilities at certain levels of fighter: +1 to hit with certain weapons, extra weapon proficiencies, some picks of niche fighter powers really geared towards a guy that knows his martial combat. These may scale off of your fighter level, where appropriate. You also get some abilities that scale off of your adventurer level, though: Bonus skill picks as "class skills" for fighters, access to some more general but higher level martial abilities, etc.
If you pick wizard instead, you get ever increasing access to some potent but niche spells (as opposed to the very common spells that any adventurer can learn). Picking both over times expands your picks, scales with your adventurer level, but damages the scaling/focus of the speciality class stuff.
I suppose that is a bit too much like turning wizard and fighter into 3E prestige classes for comfort, and I've probably drawn the example lines in bad places. You'd need a good selection of abiltiies to place in the various slots, to see if the framework would really work. I think if it did work, the class stuff would need to be kept tightly focused and narrow. And it is entirely possible that such a framework would work better by sacrificing a bit of edge flexibility in return for more clarity, via replacing the "adventurer" base with one of those handful of core bases that various people have advocated at time--warrior or caster; warrior, caster, rogue, etc.