I know this topic has been beaten to the ground. Despite arguments against it, there is no doubt that adding new specs to existing classes would add a ton of content into the game and more content is never a bad thing. I realize that balancing is an issue but I don't think it's fair to underestimate a multi-million dollar company, especially if executed right.
First off, the expansion after Legion should NOT introduce 4th specs since the Demon Hunter was just introduced. Instead the next expansion should focus on perfecting the current classes as best as possible in both mechanics and fantasy. The following expansion should then roll out the new specs. To ensure that balance is done to its maximum Blizzard could potentially release half of them during the launch and the other half during the first patch like 9.0.5, similarly with how the updated Belf models came after WoD's launch.
So my design for 4th specs would be as follows. After reaching max level, players would unlock a small quest chain to unlock their class' Paragon Specialization. Similar to character boosts, the chain would include a training tutorial for the new specs. Here are the spec's that I've thought of.
Warlock: (Blood) The Warlock manipulates life energies and blood magic to heal their allies. Healer
Mage: (Temporal) The mage uses their mastery of time to reverse wounds sustained by their friends. Healer
Priest: (Inquisitor) Now for priests I wanted to try to be a little unique and introduce a melee spec as the first melee clothie in WoW. Either they would have their own martial art like monks or utilize a one handed weapon with a held in off-hand weapon and be a mid range melee-caster spec. If these would prove to be too outlandish then a Holy based spell DPS would be their 4th spec.
Druid: Obviously Druids already have a 4th spec. However, instead of introducing a 5th spec I gave them a beefed out Heart of the Wild spell.
(Heart of the Wild): The druid activates one of their other specializations, unlocking all chosen talents and abilities as well as providing appropriate stats. Lasts 30 seconds, 30 min CD. Does not reset, like reincarnate, to prevent druid stacking.
Monk: (Flame Dancer) Utilizes a mixture of flame martial arts based on the Red Crane of Hope and a ranged weapon to deal damage. Ranged DPS (think of avatar the last airbender and their firebenders)
Rouge: (Dead eye) A ranged assassin using ranged weapons to dispatch their enemies from a distance. In addition, Outlaw is now a tanking spec. Ranged DPS
2nd option (Bard) A back alley traveling performer/con-artist that uses their various songs and melodies to heal... for a price of course. Uses ranged weapons with Int. Outlaw stays as DPS with this option.
Demon Hunter: Now before you call me crazy on my DH ideas just hear me out. In Warcraft Chronicles it is said that Fel is the result from Light and Shadow colliding into one another. So what if DH's found a way to reverse the process and create two new styles of fighting during their quest.
(Light Sworn) The Demon Hunter is imbued with pure light energy, dealing devastating and blinding-fast attacks. Melee DPS
(Void Bound) Using Two-Handed weapons, the Demon Hunter uses the power of the void to overwhelm their foes with hungering shadows. Melee DPS
Shaman: (Earthwarder) The Shaman uses their mastery over the earth to protect themselves and their allies from incoming onslaughts. Tank
Hunter: (Dark Ranger) Inspired by Lady Sylvanas, the hunter uses sinister shots to slowly bring agony to their foes. DOT spec. In addition, Survival is now a Tanking spec. Range DPS
Warrior: (Gladiator) The warrior uses a shield and a two-handed weapon for savagery over defense. Melee DPS
Death Knight: (Necromancer) The Death Knight uses their command over dead flesh to stitch-up their allies. Healer
Paladin: (Conviction) The Paladin's inner light burns hot, enabling them to unleash seething blasts of radiant light towards their foes. Ranged DPS
So that's my two cents on this dead horse. Feel free to give feed back or post your own 4th spec ideas.