Update: This post is coming from someone who plays on a lot of private servers from Classic to MoP. I'm not really familiar with most of the retail specializations because retail wow is hot fucking garbage.
There should have been an additional specialization added for each class when they decided to purge the talent trees back in MOP. If they're going to remove customization, they should have at least somewhat increased player choice. I've added below concepts for them to possibly implement, or things they could have implemented in the past. Each class here has at least four specializations, some have a fifth or sixth possible specialization. (That doesn't mean I think they deserve more than four)
Warrior
1. Arms
2. Fury
3. Protection
4. Gladiator = Sword and board dps
5. Medic - Non-magic healer, triage and combat medicine. Playstyle includes building rage based off of damage taken by self and group members, and channeled abilities.
Paladin
1. Holy
2. Retribution
3. Protection
4. Exorcism - Shockadin style ranged dps, holy damage dealer. Playstyle includes building and spending holy power(only paladin spec to use it). Heavy burst emphasis.
Hunter
1. Marksmanship - No pet.
2. Survival - Melee dps, with pet sacc option. (One will inevitably be meta, but that's okay).
3. Beast Mastery - Melee pet focused tank, 2 pets. Playstyle includes a unique approach to tanking; Shifting aggro and damage between the hunter and his pets, having pet dps and pet healers. Ultimately a strong solo tank specialization, which would fit hunter theme of solo play well.
4. Woodsman - Similar to old survival, think a magical archer. Playstyle includes dots, cc, and old survival abilities.
5. Bombardier - Tech style ranged dps. Playstyle includes explosives, traps, aoe heavy. This satisfies tinker demands without a stupid tinker class.
Rogue
1. Combat
2. Subtlety
3. Assassination - Ranged dps/Melee dps hybrid. Think sniper with knives. Playstyle includes ranged openers, gap closers, and hit and run tactics.
4. Guile - Class that plays similar to assassination
5. Control - Melee tank. Playstyle focuses on avoiding, diverting and deflecting damage. Think successful ability procs enabling evasion CD resets for example. Smoke bombs and special CC that can control even the strongest of boss fights.
Priest
1. Holy
2. Discipline
3. Shadow - Ranged, mobile burst dps. Playstyle involves shadow orbs, insanity, and mind spike spam. Reworked to expand on the old vampiric abilities, and provide off healing.Incredibly powerful in solo play, but offers a good backup support role in groups.
4. Reconciliation - Ranged holy dps. Playstyle includes a classic mage/warlock sustained direct ranged dps to provide a cloth caster that does holy damage.
5. Monastic - Melee cloth dps with intense damage mitigation. Not tankable but the priest balances inner will charges with damage output. Ability to burst as a glass cannon and give up defense, or able to sacrifice damage for a tank level damage mitigation.
Shaman - Ultimately I see two paths here: Separate a shaman by elements, or do an approach where all specs have some kind of cross elemental play. Regardless, the class would play similarly, it's a matter of aesthetics. Totems should be a serious expanded feature.
1. Restoration
2. Storm/Elemental
3. Lava/Enhancement
4. Earth/Spirit - Tank spec. Playstyle includes 2h melee, shield dancing based on need, earthquake instant procs, healing rain instant procs. The shaman tank is a tank that should be formidable in a group setting, in that he can tank and offer support to his teammates. This contrasts say a Prot paladin or blood dk, which is mostly self contained.
Mage
1. Fire
2. Frost - Expand on the water elemental, or sacc it.
3. Arcane
4. Time(Chronomancer?) - Healer spec. Playstyle inspired by stagger mechanic - Reverse damage, expunge damage, and change the course of combat. Mix in debuffs on enemies to mitigate damage on allies.
Warlocks
1. Destruction
2. Affliction
3. Demonology - Tank, similar to MOP tanking with proper glyphs.
4. Sacrifice - Hybrid healer/dps. Playstyle focuses on draining enemies, and even mixing debuffs on allies with heals, (ex: reducing an allies dps so the tank can receive more damage mitigation, or applying a life transfer between an ally and tank). Intensely strong in five mans due to chaotic playstyle, but backseat in raids. Strong in battlegrounds due to the constant dying of teammates, but weaker in arenas.
Monk - Could have been separated by celestials.
1. Windwalker
2. Brewmaster
3. Cranedancer - Fistweaving spec. Healer.
4. Mistweaver.
5. Spirit - Ranged dps utilizing a dark and light chi system. Playstyle revolves around chi bombs and mental fortitude. Agile, high damage avoidance/mitigation. Inverting the mistweaver channel style by utilizing that same style with jade lightning and revolving chi spending around that.
Druid
Duh
Death Knight
1. Blood
2. Frost
3. Unholy
4. Necromancy - Plate ranged dps. Yes I know it would be strong. I am sorry but DK's should just be strong. That is the point of them. Playstyle integrates runic system.
Demon Hunter
Cringe class, remove it from the game.