"Mystic" is the next class in alphabetical order. Since Mike Mearls said they got the article finished by the end of last year, I'm expecting it either tomorrow or next Monday.
The other most likely class would be rogue, since I doubt ranger will be in this round, as much as it is "Under Construction".
About the fluff tangent, the discussion got me thinking of a better way to explain why I don't like the "just re-fluff it" approach.
Refluffed D&D is just d20. The more I have to refluff it, the less I am playing D&D, and the more I am playing a different game built off of d20 mechanics. I'm not buying that; I'm buying official D&D products. Therefore I care about what fluff gets published. Heck, personally D&D for me is more about the fluff than the mechanics anyway.