It would be extremely difficult, remember that the MK as you know it is the second story of a two story structure. Placing another large building behind the current castle (a la Hong Kong) would place enormous strain on the basement. Also there is not much room in that back courtyard to extend the structure without intruding on the Philharmagic and Pooh show/ride buildings.If you asked that question 5 years ago I would say no, never. But after everything they've done recently I wouldn't put anything past them.
Finally, they could only extend the height by 10ft before they have to put a red airplane hazard light on the building.