It just means the budget will be above $100 million. I find it incredibly unlikely Legendary Pictures or Blizzard has let anyone connected to the media know exactly what the plot it going to be.
All in all, I wouldn't exact any WoW expansion to be directly adapted since they tend to be continuations of story arcs created in the past. They might be best off if they forgo any direct game adaptation and set it between games.
Metzen hinted at BlizzCon that the story will be familiar, so I don't think it'll be something completely out of the blue. If I could hazard a guess, I wouldn't be surprised if we have an Of Blood and Honor adaptation. The idea of an old war veteren coming to the realization that the enemy he has fought his entire life is more complex then he had originally believed, all set to the backdrop of the beginning of the scourge destruction of Lordaeron.
If it's successful enough, it could spawn a sequel beginning with the collapse of Lordaeron from Tirion's perspective and finally end with him rebuilding the Knights of the Silver Hand. If the sequel is successful, then we could have a WotLK adaptation, which could end with a nice flashforward to Tirion as the de facto king in a restored and Argent Crusade controlled Western Lordaeron.