Just a little bored and wanted to be semi-creative. I know that this game is designed for the Trio specs of Tank, Healer and DpS. But if they were to add in some support specs, I'd actually like to see them being tossed in for the classes that currently only got DpS. Just to change one of their current dps specs into them. Keep in mind some of these things would be a bit broken in PvP but, going for a PvE point of view for now. They will still have damage dealing abilities, but they will obviously be less than the DpS specs.
Warlocks: Using Affliction to CC and curse. Fear is used as a CC tool, with less diminishing returns and can fear up to 3 targets. They use curses to debuff their enemies: Weakness to reduce their damage dealt, tongues to slow their spells down, fatigue to reduce their auto-attack and movement speed.
Mages: Using Arcane to CC and buff. Like the warlock, they can Polymorph, just for longer and more of them - and reduce the amount of health/mana they regain. They use their arcane arts to buff other players' spells and attacks to deal extra Arcane damage with a short temporary buff, and they can use time-related spells to speed their attacks and movement.
Rogues: Using Subtlety to CC and 'cripple'. Unlike other rogues who can only stealth outside combat, apart from Vanish, these rogues can do it while in combat - but their stealth has a longer cooldown for it. All their support is used via stealth abilities: They can still only sap out of combat, but since they can enter stealth repeatingly in a fight, their Cheap Shot is a stun that lasts a bit longer than normally. They also use things such as Expose Armour to allow others to deal more damage on the foe, or do a bleed effect that reduces their movement and auto-attack speed as they take damage over time. And if the enemy has a wepaon, they can disarm them for a while.
Another thing they can do is provide a better AoE-stealth effect for allies, and if talented, whenever they enter stealth they can bring one other person into stealth for a short while, which can reset their aggro and their next attack deals bonus damage.
Hunters: Using Beast Mastery to CC and pet-support. They use their traps to snare, stun or slow down in an AoE. They can also do short-duration taunts in order to trick them into the traps. Their pets will be able to support in a lot of different ways, depending on which pet it is. A more 'tanky pet' which is a bit more reliable, or one that roars to frighten enemies, or 'bind' themselves to a random player to increase their damage, or shred an enemy's armour off for overall extra damage. A mix of the other support roles, and all depending on their pet.
And minor thing for PvP, though more of a tank thing kind of support: Allow tanks to taunt players. It just forces them to target the warrior for like two seconds, but might be able to save a life - unless the player is capable of using AoE, thus putting that stuff into more use.
It's not the best list, but felt like sharing the ideas at least since I think there's some potential in it. I'm pretty curious, though, what the rest of you would like to have from support roles if they would become a thing (even if they probably won't)!