I think that one of the major problems for a WoW2 is that this "new Blizzard" (i.e. an Activision's subsidiary) has notoriously struggled with the new installments for its other two big franchises: Diablo and Starcraft. D2 was a big success, D3 is a dead game. SC1 was an ever bigger success, while SC2 is on life support. Which is a shame, I actually think it's a damn good game, unlike D3.
Now, imagine how nervous will the bigwigs at Blizzard become if someone ever proposes a new installment for WC/WoW...