I don't think any CLASS needs a complete overhaul, but I do think we could do with some fundamental spec changes.
I'd especially like to see more tank specs, for various reasons. Pure DPS classes in particular could fairly easily accommodate a tank spec to replace one of their three DPS specs.
Survival Hunter could easily be turned into a tank (I mean it's literally called SURVIVAL after all
). Bombs and stuff could remain, and just be modified into tank abilities. Come up with an interesting defensive mechanic, and you're golden. Flavor is easy to do, just lean on the animal theme and do Aspect of the Clefthoof or whatever.
Demonology Warlock as a tank is something I'd personally LOVE to see. Give them an ability that effectively swaps the player control to the pet, turning the "player" into a pet. That way you can easily maintain the pet fantasy, but not worry about issues with movement etc. because you'd be in direct control. Tons of options there, including using other summoned demons for defensive purposes (taking hits for you, sharing damage, etc.) to maintain the spec fantasy.
I'd probably also switch Outlaw Rogue to a tanking spec. The pirate theme can easily stay, and just be retooled into the tough scoundrel who just won't die. Lots of CC options, and a bag of tricks of all sorts for defensive purposes - there's already a lot of familiar names there that could support that flavor, like Riposte, Evasion, Smoke Bomb, etc. all of which have defensive connotations. That would have the added benefit of freeing up a more defined role for Subtlety, so you could leave Assassination as the poison/bleed DoT spec and have Subtlety as the direct damage/burst spec.
Mage would be the hardest of the pure DPS classes to redesign. I could see both Arcane or Frost fill a tanking role, but I'd probably lean towards Frost because it has an inherent control theme and some interesting defensive flavor with ice-based barriers/shields. This could give interesting options to a tank with controlling mob movement, while retaining the idea of a tough target covered in layers of ice. In fact, some sort of frozen shell could be the main defensive mechanic, trying to maintain a number of these shell stacks as a regular rotation. Iconic mage abilities like Mirror Image could be repurposed as defensive cooldowns. Much like the Rogue, this could solidify the niches of the two other specs, with Fire being DoT/AoE based and Arcane being bursty.