Two things!
First, look at the area to the left of where the wording of "Studio Backlot Tour" is and above where Catastrophe Canyon presently sits. It's a pond (which can be moved) storage buildings, parking lot and other not revenue producing junk. That alone doubles the area available.
Second, they are already or have already set plans to change the entrance to the park to the far right of this picture. In other words the current access road and beyond can also be used for expansion. Yea, I know, wetlands, blah, blah, blah. Well, Disney gave them, Disney can taketh away. That creates another large parcel that can be used for expansion. In my mind, it was always the most logical area to do stuff in when redoing DHS.
What they will put there is anybodies guess at this point. Probably a couple of spinners, a pin shop and a DVC sales booth, but, they can do a lot with the place if they decide to and do it without taking out anymore attractions.