Out of all the new class ideas, the most likely next class will more than likely be the Runemaster. Here's why;
1.Already exists in WoW lore, and they can be of many different races with little adverse effect to that lore.
2.WoW lacks a monk-type class, and the monk class is an extremely popular archetype in RPGs. It would go a long way towards boosting interest in WoW, which would be 8 years old by the time the next expansion arrives.
3. Runemasters can fit into any expansion. Unlike Demonhunters that seem more themed for the Burning Crusade, or Wardens who would be better themed for an Emerald Dream type expansion, Runemasters could be employed to fit any expansion blizzard is going for.
4.The Runemaster class can easily fit any role type. So if Blizzard needs a tanking, healing, or DPS class, the Runemaster could easily fit their population needs. Not only that, but the Runemaster's mechanics could go a long way towards shaking up roles in dungeons, raids, and groups. Runemaster healing would more than likely be far more dynamic than caster healing for example.
5.A lot of WoW comes from Diablo. Of all the classes in Diablo, only the monk lacks a true counterpart in the WoW universe. Of course, the class wasn't added until D3, but the point remains.
6. Personal opinion: Fist weapons are underutilized, and it would be cool to see a class that uses fist weapons, or unarmed combat of some type.
In the end, we're about 8 months away from any news about the next expansion, and about 20-23 months away from the next expansion. Here's hoping that Blizzard makes the announcement that a lot of people are waiting for.