I've voted Ability scores (though, I'd thrown out ability scores and retained only modifiers), HP, Great Wheel and Levels. Also I'd add that progression should be drastic -- like from a human to a god.
As for classes, I'd return to Fighter, Magic User, Specialist and Cleric -- everything else can neatly fit as subclasses. I don't see any point in having Barbarian and Paladin as distinct classes, for example -- they are basically fighters, but with a bunch of extra ribbons.
If all of the flavor is moved to the subclass, what is the point of classes? I mean, fighters get skills, clerics use magic, but subclasses get abilities that change the way an entire class plays? What role do classes serve under such a structure?
I ask because I've seen a lot of people advocate for fewer classes, and the best answer they can give is "it's simpler" - but if you lose variety, is that really an improvement? And if you don't r=lose variety, you've added complexity in the form of another level of taxonomy.
Edit: actually, there is one scenario I can think of where collapsing classes like this makes some sense. If you assume all magic works the same way, having different classes of spellcasters is less logical. But that would mean: if you assume that a wizard, a cleric, and a bard casting
detect magic are all doing the same thing: same gestures, same words, same components, same underlying energies being manipulated in the same way - then, yes, the difference between these is not 'what they do' but 'where they learned to do it. One class called "magic-user" with subclasses like wizard, cleric, bard, druid, warlock, sorcerer, and psion makes sense. But in that case, you wouldn't have spell lists, because if you can learn to cast
fireball you can learn to cast
cure wounds, and vice versa. There would be one master list everyone uses, with some subclasses having bonuses to particular types of spells. (ie life clerics are
better healers, but anyone who can cast 1st-level spells can cast
cure wounds.)
If, on the other hand a wizard and cleric are using two different types of power: why are there only two? Why are druids the same as clerics? Why are bards the same as wizards (but can somehow break limitations wizards must face in terms of what they can theoretically learn)? Why not have a new class for each new way of doing magic? The 4e "power sources" didn't break the lore, they doubled down on the direction the game had been going since druids were introduced, if not since clerics.
And following from that: non-spellcaster classes should not be dramatically more or less broad than spellcasters. Barbarians aren't just fighters from a low-tech-infrastructure environment, they have a distinct
way of fighting that captures different power. Maybe, if all spellcasters are one class (1), then all fighters should be one class. But I'm not convinced of the former
at all, for a game that wants to brand itself Dungeons and Dragons.
(1) or a few classes, since under this theory of magic you'd only have three base classes, which means it might be easier to do half-classes as new classes rather than dealing with multiclassing.