I thought of this just now and it probably isn't original at all. However it is an idea I was fond of so I thought to bring up it to you guys.
Take the warlock. Take a lot of people who would rather play a necromancer. Instead of making a new class, mechanics, etc. it would be simple to make a "skin" for the warlock that completely overhauls the aesthetics of their spells and minions. For example the Imp would become the Skeleton Mage, the Succubus could possibly become the Shade, etc.
Nothing would be altered mechanically, only the aesthetics. Of course something like this would take time to produce so it would make for a good item to buy on the Blizzard Store. However it wouldn't cost too many new resources, since much of the process would only include swapping icons, models, spell effects, etc.
Essentially it would allow people to play as certain popular professions which couldn't be made into brand new classes but could be incorporated as "skins" for the existing classes. Not every class would have a skin either, and some may never see one although it is likely that there is room for every class to receive a skin.