The only real feasible thing they could build in the space they own would be a waterpark.
However, given the climate, it wouldn't be able to be a year round operation, UNLESS they enclose it, which I suppose is possible, but would be pretty costly.
There are a few waterparks around here (Wild Rivers, Knott's Soak City, Six Flags Hurricane Harbor) and they all operate on a seasonal basis only.
However, I think a Disney waterpark in Anaheim would be awesome, and TOTALLY a draw, considering that, in the immediate vicinity to Disneyland resort, there is nothing like that available.
INTERCOT Staff: Disneyland Resort-California, The Water Cooler
Wilderness Lodge Aug. 7-15