Barbarian: The 4E version does a solid job, as I've expressed a dislike for the Pathfinder version. A cross between that and the Warblade (recovery mechanic and a variation on the class features) would do wonders.
Druid: Split into two classes. I'm working on a Lycanthrope-inspired Totemist in my free time (in addition to everything else), so that can cover Wild Shape. The spellcasting part should be identical in design to the Wiz/Sor (low HD, Poor BAB, poor Fort and Ref saves, 2+Int modifier skills). The individual spells need to be modified (a nightmare on it's own).
Cleric: Cloistered variant is forced. Poor HD and Poor BAB should be something common to all full casters, not just Sor/Wiz. A majority of the Personal range spells can be transfered to the Paladin's list. All spells need to be reworked (again, nightmare).
Sorcerer: Beguiler/Dread Necromancer/Warmage all replicate the Sorcerer's abilities, and the flavor can be transfered between the classes easily. Combining those three and making additional ones for the other options (Abjuration/Divination should be one class, Transmutation should be it's own, and Conjuration has enough support to be it's own class). Spells are the major offender here, so again a revamp is needed.
Wizard: Spells need to be fixed, and the Familiar needs to be somewhat fixed/nerfed.
Vanican casters in generla: I feel that true Vanican casting should be reserved for classes like the Bard, Paladin, Hexblade, and others, and that full casters need an entirely new mechanic. Those classes show that Vanican magic can be made to work, but it needs extreme limitations on it.
Bard: Fix Bardic Music so it doesn't take 6 skills just to use. Fix some major offender spells (Celerity, Glibness, a bunch of others).
Ranger: I feel this class should be given a group of variants. Version 1 would be the Sublime Variant that tempest Stormwind gave us all (I'm working on finishing it). Version 2 should be a Duskblade-esqe caster. Version 3, if we decide to keep Wild Shape at all, should be the Wild Shape variant. Two of the three should have the Scout's Skirmish ability, while the Wild Shape needs a mechanic of it's own. Favored Enemy should be dropped. The Scout is going to need an overhaul, though a fix to the action economy and the way combat works would be enough to fix it almost entirely.
Why? The Sublime variant has been compared to a Tier 3 class often enough to be Tier 3 itself (the base chassis is the Warblade, after all). The Wild Shape variant is Tier 3 by default. The Duskblade is somewhere between Tier 4 and Tier 3. Making the class into three options would give us enough flexibility to work with, while at the same time ensuring that the class is up to par. Balancing the three options would be all that remains.
Fighter: And the endless cycle begins. I've pointed out the major flaws many times now, but if I need to repeat myself I will. The thing is, so many fixes are focused on the bonus feats that they overlook actually fixing the feats in the first place. I've seen people subsume bonus feats into the class itself (a bad idea), I've seen minor statistical boosts (the majority of all fixes, none of which fix anything other than minor numbers). This is going to need time, but we need to make sure we are all clear on the major problems.