I think they have to do at least something.
Otherwise, why Blizzcon this year? They had another SC (HotS) and WoW (MoP) last year, and that wasn't enough... that's all they're known to have this year (Protoss and possibly a pretty big-picture/edgame WoW)
I mean, I took making it non-annual to mean: If we're saying something, it's big. What else makes this big?
Could also just be that they don't want to go two years without, though. Who really knows till it's time?