So I thought of this at work today and it seems, idk, plausible? In all the classes, there seems to be a running theme of one 'Good' class, one 'Neutral' class, and one 'Evil/Darker' class for each armor type. This may be a bit of a stretch, but I think it wouldn't be a horrible assumption to assume the next class is going to be darker. Think about it.
For Cloth, Warlocks are 'Bad,' Mages are 'Neutral' and Priests are 'Good'
For Leather, Rogues are 'Bad,' Druids are 'Neutral' and Monks are 'Good' (Druids and Monks are arguably interchangeable.)
For Mail, There is no 'Bad' class, Hunters are 'Neutral' and Shamans are 'Good'
For Plate, Death Knights are 'Bad,' Warriors are 'Neutral' and Paladins are 'Good'
So by that logic, the next class will be a mail armor wearing and evil/dark themed.