I'm not a WOW player...never had been and never will. But, it seems to me that their problem is not content but changing content. I recall at least 20 "patches" for WOW which drastically changed the balance of the DLC so much that everyone must have become frustrated trying to figure out the changes.
The other thing I noticed when I watched a friend play WOW was that the graphics have lots of high resolution art but for the most part the majority of the textures are solid colors...like toys. I think if Blizzard wants to rejuvinate their player base and turn it around then they will need to develop an entirely new look...more artsy-crafty and less toy-like...for all the levels.
An example would be to wonder how players would feel if they bought Duke Nukem Forever only to find that they have to play through the original Duke 3D games before they are allowed to have fun with the new content.
Let's Rock!