Here’s my thoughts on my 3 most played classes (DH/Wiz/WD). This is in response to the OP though to save space I won’t quote him.
Demon Hunter
Shadow: This should be reworked to buff Fan of Knives and Chakram as well. Chakram would work similar to impale, but to accommodate FoK you need area-of-effect to work with the set. A bracer could work, but a better idea is to rework the 6p bonus to deal damage that scales inversely with your distance from the target.
UE: agreed on numbers-only buff plus buffing multishot’s supporting legs
Marauder’s: transfer the Zoey’s Secret effect to the 4p somehow. Otherwise agreed numbers-only buff
Natalya’s: this should be reworked to be about Vengeance instead of Rain of Vengeance. Having the 2p bonus reduce the cooldown of Vengeance, for instance, would make Dawn unnecessary for Nats.
Witch Doctor
Disagree with your overall assessment of the class. WD sets are actually in a very good place design-wise, in that each set accommodates a different skill (Mundunugu SB, Arachyr firebats, Zuni darts, etc.). Contrast this with Hungering Arrow, Tempest Rush etc. Furthermore every set has an engaging playstyle, if played correctly. What the class really needs are numbers buffs, such as:
Zuni: needs more dmg reduction, and a rework of the clunky 6p bonus which only affects enemies hit by mana spenders
Jade: agreed numbers only, but Quetzalcoatl is in no way ‘mathematically-confusing’ provided you carefully read the 6p bonus. By compressing the DoT duration by half it is essentially doubling our dmg (But you don’t have to understand this to actually play the build).
Helltooth, Arachyr: only needs damage and dmg reduction buffs, playstyle-wise they are fine
Mundunugu: actually one of the most polished sets in the game, it just needs its ‘spirit barrage no longer counts as a pet’ nerf reversed
Wizard
Firebirds: would be fine provided it is divorced from mirror images. I need not say more.
Tal Rasha: while many advocate a rework for meteors as that was its intended skill, I prefer this set to be skill-agnostic, as it is the only one Wizard has. And the ‘mental constraint’ of juggling 4 stacks may be artificially imposed by the OP rather than the set itself. Agreed that the 4p dmg reduction should be reworked into %dmg reduction. Besides that, it also needs a hefty dmg buff.
DMO: only needs damage buffs, playstyle-wise it’s fine, and should definitely NOT be made into a close-range set. If you’re playing DMO frozen orb correctly you should be able to intersperse Slow Time casts between your frozen orbs.
Vyr: agreed Chantodo is overdone. The best solution is to buff Manald Heal and turn this into the lightning wiz set.