Create several new servers. These servers would be labeled "Hard Mode" Pve or Pvp.
Offer free transfers to this server for a limited time.
Increase every encounter difficulty by 50%. or scaled to what ever blizzard would find to be a hard challenge.
The players who cry about the game being to easy, could simply go to that server and play. Questing would be harder, Bosses would be extremely harder and so forth. Even gold would be harder to obtain.
They want a difficult game, why not give it to them.