Thought one:
Some classes seem to change roles, as the game is currently played.
Let's look at the fighter and the wizard.
At low levels, the fighter can very effectively hold back one opponent, and the wizard is in charge of crowd control.
At high levels, the fighter is far better at managing multiple opponents, and the wizard is far better at taking out single opponents.
Thought two:
When looking for who's in and who's out, my speculation runs towards how classes can be categorized:
Martial ability (BAB 1, 3/4, or 1/2)
Caster ability (spellcaster levels, and divine or arcane casting)
Skill points (2, 4, or 8)
If you think of these as three connected sliders, moving one up generally moves the other two down.
There's a potential 4th slider, Civilized vs. Wild, but it's really more of a toggle. The Wild classes generally have a more limited selection of spells (nature-based), skills, and equipment, but it seems in exchange they get a couple of extra skill points. (Fighter vs. Barbarian, Ranger vs. Paladin, Druid vs. Cleric).
Anyhow, if I were doing the redesign I would look at those basic classifications and see where there are holes that could be filled (where the bard used to be, I guess...), where there is redundancy that could be eliminated (sorcerer), and which classes don't fit into this design framework (monk).