Draele over at Rantings of the Afflicted asks if WoW could ever have what he calls a "meta" class. He lists the examples of Mesmer in Guild Wars or a Psionicist in Vanguard, and I'll add to that the Sorcerer class in Dark Age of Camelot-- classes that depend on manipulation and mind control rather than direct damage or healing.
I'd say that it's possible, but extremely unlikely. Why? Because WoW is based on an RTS game, and in RTS, there's not a lot of complicated manipulation going on-- either you're attacking or defending, or some mix of the two. There hasn't really been any precedent (that I can think of or stretch to) in the Warcraft universe for a Bard or "Mesmer" class, and that's why it's pretty unlikely that Blizzard will try to break out past the trinity of usual MMO archetypes. Not to mention that, as Draele says, a meta class is a complex thing to create and play, and WoW tends to be more casual than complex.
Of course, never say never. There's a lot in this universe that hasn't even been hinted at in the game yet, and as was mentioned in last week's podcast, Death Knights will likely only be the beginning of hero classes, so who knows what Blizzard could come up with.