Most of the fixes that work have already been done, which is why I just ban things. All keeping the classes around does is keep the metagame tags, which are unnecessary if you're making your own character.
The biggest problem with the Big Five (except Artificer) is that they can learn every single spell in the universe, and that number of spells is vast. If you don't do away with that, you will never
balance the Big Five without nerfing the casters who are already balanced because they simply have too many meaningfully powerful abilities when you give them every spell. The same applies to Druid forms.
In the case of the Wizard, you can go through and cull the Wizard list so that even if they do get all the spells in the game, they aren't the dominating force in the game (at which point you may as well use Wu Jen). You can convert them to spontaneous casters with the spells known limit to go along with it (at which point you may as well use the Generic Spellcaster).
Cleric: Spontaneous Divine Caster
, first and foremost. Either remove or cull divine feats, so their Turn Undead can't fuel the worst of the lot, most notably Divine Metamagic. The result is basically a Favored Soul. If that's not enough for your taste, force the Cloistered Cleric
Druid: Spontaneous Divine Caster
, first and foremost. Wildshape gets replaced with the Shapeshifting variant from PHB2. That ought to cut it, though in some games it may do to replace or remove the animal companion.
Archivist: You can probably just do Spontaneous Divine Caster
from both the Cleric and Druid lists, without access to domains or every other divine caster in the universe.
Artificer: So not worth it.