We could really use an actual Void based class instead of just having 1 spec use it. The Void is basically 1 of 2 ultimate powers in the WoW universe and we already have multiple classes and specs that can use the Light, I think the Void deserves an entire class dedicated to it and since 9.0 is shaping to be a Void Lord kind of expansion, this would be a perfect time to add one.
It would have 3 specs:
Melee DPS: My favorite style of melee has always been the high mobility large weapon wielding type, which WoW doesn't have at the moment. Something akin to the Blade Dancer (sort of like combining the big 2h bad assery of a Warrior with the swift mobility play style of a DH/Monk).
Ranged DPS: Ranged DPS are all being overshadowed by melees right now and a new ranged spec would be needed for the new class. Again, keeping in line with the mobility theme we'd have a slippery glass cannon style ranged class like an Arcane/Fire Mage, however, instead of relying on iceblock/cc to survive, this one will be able to self cast heals on themselves like all hybrid classes and also because...
Healer: Another useful spec to have. All the previous newly added classes (DKs/Monks/DHs) all came with a tank spec, we don't need more of those. This class will have no tank spec, but a healing one instead. Since we already know void can also heal (SP's leech and Disc's Shadow Mend) we can make this healer apply those two style of healing. Make it a DPS leech style of heals similar to Discs but also keeping it in theme with high mobility and lower mitigation.
No point in suggesting specific abilities, but those general themes would be unique to anything we have and pretty cool.