They added new races.
Added the DK.
We now have new models coming.
A new class would entail a lot of work and balancing, and we already have classes and specializations (sub-classes), so there's enough variety there.
We have many races, but IMO lack enough customization.
Sub-races would fill the niche of something new that doesn't require a ton of work and balancing.
They already have many of the skins in place with NPCs, like the brown orcs and the taunka.
To further differentiate the sub-races they could also add in a new animation or two and a slight change to the racial abilities.
This would be a benefit to both factions and be a lot of fun. They should do this next IMO.