There is so much potential in game for so many classes that it would be mad for blizzard to not seize them all and treat the fans with new ways to play the game and attract newcomers.
But to answer directly to the OP, I think what will come next is a demon hunter for sure,taking its place on the conqueror's tokems and using mail armor.
Next we might get a Tinker, but it will use leather and begin the leather classes lead anew. By then, we will be in the South Seas fighting Azshara and Blizzard will hopefully have added the support class. Next to the Tinker, we need a cloth class, and for that role, I think the Invoker is the best on set because its a good change from the normal mage, since even though it is in essence a mage, it's mechanics are more complex to master in contrast to the mage.
In another expansion, we could have the Dragonsworn to play its part on the defense on Azeroth, bringing the dargons back to action with renewed strength. This class would use mail. Finally, my 4th choice of a class is the battlemage, which is the sum of a mage with a plate armor, this way it all comes together nicely =).
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Had enough of Dragons for the remainder of WoW, personally.
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