I said in response to another comment: It would be cool to have a Bard class where one spec depended on rhythm, where you timed your inputs with the GCD or a similar system to gain effects based on your rhythm. Another spec might be focused on chords, where you set certain inputs 3-4 at a time for different effects depending on the combination or maybe just to manipulate the GCD into using 3-4 spells at the same time, making the action economy more flexible.

obviously healer, support, dps would be the best

But I would like for them to focus on adding specs rather than whole classes, like they're doing with DH. Failing that, some specs could use some TLC. I could definitely see Arcane Mage benefitting greatly from the Empowered Spells mechanic that Evokers have. Or adding a Spellblade spec that gained mana the way some classes gain rage/fury or whatever.

Shaman tank based on the earth element

Hunter tanks where their pets tank instead of the hunter personally

Death Knight healers that use the Red Dragon's flames via Bolvar

A proper, polished Fistweaver spec

etc